feat(Core/Events): Add dynamic holiday date calculator (#24038)

Co-authored-by: blinkysc <blinkysc@users.noreply.github.com>
Co-authored-by: sudlud <sudlud@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

data/sql/updates/pending_db_world/rev_1767341430742537600.sql

@@ -0,0 +1,30 @@
+-- correctly link brewfest building event in ironforge to brewfest
+UPDATE `game_event` SET `holiday` = 372, `holidayStage` = 1 WHERE `eventEntry` = 70;
+
+-- remove start and end times from automatically handled events
+UPDATE `game_event` SET `start_time` = NULL, `end_time` = NULL WHERE `eventEntry` IN
+(
+    1,  -- Midsummer Fire Festival
+    2,  -- Winter Veil
+    3,  -- Darkmoon Faire (Terokkar Forest)
+    4,  -- Darkmoon Faire (Elwynn Forest)
+    5,  -- Darkmoon Faire (Mulgore)
+    7,  -- Lunar Festival
+    8,  -- Love is in the Air
+    9,  -- Noblegarden
+    10, -- Children's Week
+    11, -- Harvest Festival
+    12, -- Hallow's End
+    23, -- Darkmoon Faire Building (Elwynn Forest)
+    24, -- Brewfest
+    26, -- Pilgrim's Bounty
+    50, -- Pirates' Day
+    51, -- Day of the Dead
+    70, -- Brewfest Building (Iron Forge)
+    71, -- Darkmoon Faire Building (Mulgore)
+    72, -- Fireworks Spectacular,
+    77  -- Darkmoon Faire Building (Terokkar Forest)
+);
+
+-- drop unused table
+DROP TABLE IF EXISTS `holiday_dates`;

src/server/database/Database/Implementation/WorldDatabase.cpp

@@ -98,7 +98,6 @@ void WorldDatabaseConnection::DoPrepareStatements()
     PrepareStatement(WORLD_SEL_GAME_EVENT_BATTLEGROUND_DATA, "SELECT eventEntry, bgflag FROM game_event_battleground_holiday", CONNECTION_SYNCH);
     PrepareStatement(WORLD_SEL_GAME_EVENT_POOL_DATA, "SELECT pool_template.entry, game_event_pool.eventEntry FROM pool_template JOIN game_event_pool ON pool_template.entry = game_event_pool.pool_entry", CONNECTION_SYNCH);
     PrepareStatement(WORLD_SEL_GAME_EVENT_ARENA_SEASON, "SELECT eventEntry FROM game_event_arena_seasons WHERE season = ?", CONNECTION_SYNCH);
-    PrepareStatement(WORLD_SEL_GAME_EVENT_HOLIDAY_DATES, "SELECT id, date_id, date_value, holiday_duration FROM holiday_dates", CONNECTION_SYNCH);
     PrepareStatement(WORLD_DEL_GAME_EVENT_CREATURE, "DELETE FROM game_event_creature WHERE guid = ?", CONNECTION_ASYNC);
     PrepareStatement(WORLD_DEL_GAME_EVENT_MODEL_EQUIP, "DELETE FROM game_event_model_equip WHERE guid = ?", CONNECTION_ASYNC);
     PrepareStatement(WORLD_SEL_GAME_EVENT_NPC_VENDOR, "SELECT eventEntry, guid, item, maxcount, incrtime, ExtendedCost FROM game_event_npc_vendor ORDER BY guid, slot ASC", CONNECTION_SYNCH);

src/server/database/Database/Implementation/WorldDatabase.h

@@ -104,7 +104,6 @@ enum WorldDatabaseStatements : uint32
     WORLD_SEL_GAME_EVENT_BATTLEGROUND_DATA,
     WORLD_SEL_GAME_EVENT_POOL_DATA,
     WORLD_SEL_GAME_EVENT_ARENA_SEASON,
-    WORLD_SEL_GAME_EVENT_HOLIDAY_DATES,
     WORLD_DEL_GAME_EVENT_CREATURE,
     WORLD_DEL_GAME_EVENT_MODEL_EQUIP,
     WORLD_SEL_GAME_EVENT_NPC_VENDOR,

src/server/game/Events/GameEventMgr.cpp

@@ -21,6 +21,7 @@
 #include "DisableMgr.h"
 #include "GameObjectAI.h"
 #include "GameTime.h"
+#include "HolidayDateCalculator.h"
 #include "Language.h"
 #include "Log.h"
 #include "MapMgr.h"
@@ -34,7 +35,7 @@
 #include "WorldSessionMgr.h"
 #include "WorldState.h"
 #include "WorldStatePackets.h"
-#include <time.h>
+#include <chrono>
 
 GameEventMgr* GameEventMgr::instance()
 {
@@ -1070,51 +1071,126 @@ void GameEventMgr::LoadFromDB()
 
 void GameEventMgr::LoadHolidayDates()
 {
-    uint32 oldMSTime = getMSTime();
+    uint32 const oldMSTime = getMSTime();
+    uint32 dynamicCount = 0;
+    uint32 dbCount = 0;
 
-    WorldDatabasePreparedStatement* stmt = WorldDatabase.GetPreparedStatement(WORLD_SEL_GAME_EVENT_HOLIDAY_DATES);
-    PreparedQueryResult result = WorldDatabase.Query(stmt);
+    // Step 1: Generate dynamic holiday dates based on current year
+    std::chrono::system_clock::time_point const now = std::chrono::system_clock::now();
+    std::time_t const nowTime = std::chrono::system_clock::to_time_t(now);
+    std::tm localTime = {};
+#ifdef _WIN32
+    localtime_s(&localTime, &nowTime);
+#else
+    localtime_r(&nowTime, &localTime);
+#endif
+    int const currentYear = localTime.tm_year + 1900;
 
-    if (!result)
+    for (auto const& rule : HolidayDateCalculator::GetHolidayRules())
     {
-        LOG_WARN("server.loading", ">> Loaded 0 holiday dates. DB table `holiday_dates` is empty.");
-        return;
-    }
-
-    uint32 count = 0;
-    do
-    {
-        Field* fields = result->Fetch();
-
-        uint32 holidayId = fields[0].Get<uint32>();
-        HolidaysEntry* entry = const_cast<HolidaysEntry*>(sHolidaysStore.LookupEntry(holidayId));
+        HolidaysEntry* entry = const_cast<HolidaysEntry*>(sHolidaysStore.LookupEntry(rule.holidayId));
         if (!entry)
         {
-            LOG_ERROR("sql.sql", "holiday_dates entry has invalid holiday id {}.", holidayId);
+            LOG_INFO("server.loading", ">> Holiday {} not found in DBC - cannot set dynamic dates", rule.holidayId);
             continue;
         }
 
-        uint8 dateId = fields[1].Get<uint8>();
-        if (dateId >= MAX_HOLIDAY_DATES)
+        // Special handling for Darkmoon Faire - needs multiple dates per year (4 occurrences)
+        if (rule.type == HolidayCalculationType::DARKMOON_FAIRE)
         {
-            LOG_ERROR("sql.sql", "holiday_dates entry has out of range date_id {}.", dateId);
+            int const locationOffset = rule.month;
+            std::vector<uint32_t> const dates = HolidayDateCalculator::GetDarkmoonFaireDates(locationOffset, currentYear - 1, 4, rule.offset);
+
+            uint8 dateId = 0;
+            for (auto const& packedDate : dates)
+            {
+                if (dateId >= MAX_HOLIDAY_DATES)
+                    break;
+
+                entry->Date[dateId++] = packedDate;
+                ++dynamicCount;
+            }
+
+            // Darkmoon Faire lasts 7 days (168 hours) - set Duration if not already set
+            if (!entry->Duration[0])
+                entry->Duration[0] = 168; // 7 days in hours
+
+            auto itr = std::lower_bound(ModifiedHolidays.begin(), ModifiedHolidays.end(), entry->Id);
+            if (itr == ModifiedHolidays.end() || *itr != entry->Id)
+                ModifiedHolidays.insert(itr, entry->Id);
+
             continue;
         }
-        entry->Date[dateId] = fields[2].Get<uint32>();
 
-        if (uint32 duration = fields[3].Get<uint32>())
-            entry->Duration[0] = duration;
+        // Generate dates for current year + 2 ahead (year capped at 2030 due to 5-bit client limitation)
+        for (int yearOffset = -1; yearOffset <= 2; ++yearOffset)
+        {
+            int const year = currentYear + yearOffset;
+            if (year > 2030)
+                break;
+
+            uint8 const dateId = static_cast<uint8>(yearOffset + 1);
+            if (dateId >= MAX_HOLIDAY_DATES)
+                break;
+
+            uint32_t const packedDate = HolidayDateCalculator::GetPackedHolidayDate(rule.holidayId, year);
+            entry->Date[dateId] = packedDate;
+
+            // Debug: decode and log the date
+            std::tm const date = HolidayDateCalculator::UnpackDate(packedDate);
+            LOG_DEBUG("server.loading", ">> Holiday {} Date[{}] = {}-{:02d}-{:02d}",
+                rule.holidayId, dateId, date.tm_year + 1900, date.tm_mon + 1, date.tm_mday);
+
+            ++dynamicCount;
+        }
 
         auto itr = std::lower_bound(ModifiedHolidays.begin(), ModifiedHolidays.end(), entry->Id);
         if (itr == ModifiedHolidays.end() || *itr != entry->Id)
-        {
             ModifiedHolidays.insert(itr, entry->Id);
-        }
+    }
 
-        ++count;
-    } while (result->NextRow());
+    // Step 2: Check game_event.start_time for overrides (allows custom servers to override calculated dates)
+    // Only use as override if start_time year >= current year (ignore old static dates)
+    QueryResult result = WorldDatabase.Query("SELECT holiday, UNIX_TIMESTAMP(start_time) FROM game_event WHERE holiday != 0 AND start_time > '2000-12-31'");
 
-    LOG_INFO("server.loading", ">> Loaded {} Holiday Dates in {} ms", count, GetMSTimeDiffToNow(oldMSTime));
+    if (result)
+    {
+        do
+        {
+            Field* fields = result->Fetch();
+
+            uint32 const holidayId = fields[0].Get<uint32>();
+            HolidaysEntry* entry = const_cast<HolidaysEntry*>(sHolidaysStore.LookupEntry(holidayId));
+            if (!entry)
+                continue;
+
+            if (fields[1].IsNull())
+                continue;
+
+            time_t const startTime = fields[1].Get<uint64>();
+            if (startTime == 0)
+                continue;
+
+            std::tm const timeInfo = Acore::Time::TimeBreakdown(startTime);
+
+            int const year = timeInfo.tm_year + 1900;
+            // Only override if start_time is current year or later (ignore old static dates)
+            if (year < currentYear || year > 2030)
+                continue;
+
+            // Pack the date in WoW format and override Date[0]
+            uint32_t const yearOffset = static_cast<uint32_t>(year - 2000);
+            uint32_t const month = static_cast<uint32_t>(timeInfo.tm_mon);
+            uint32_t const day = static_cast<uint32_t>(timeInfo.tm_mday - 1);
+            uint32_t const weekday = static_cast<uint32_t>(timeInfo.tm_wday);
+            entry->Date[0] = (yearOffset << 24) | (month << 20) | (day << 14) | (weekday << 11);
+
+            ++dbCount;
+        } while (result->NextRow());
+    }
+
+    LOG_INFO("server.loading", ">> Loaded {} Holiday Dates ({} dynamic, {} game_event overrides) in {} ms",
+        dynamicCount + dbCount, dynamicCount, dbCount, GetMSTimeDiffToNow(oldMSTime));
 }
 
 uint32 GameEventMgr::GetNPCFlag(Creature* cr)
@@ -1926,7 +2002,7 @@ void GameEventMgr::SetHolidayEventTime(GameEventData& event)
         }
         else
         {
-            // date is due and not a singleDate event, try with next DBC date (modified by holiday_dates)
+            // date is due and not a singleDate event, try with next DBC date (dynamically calculated or overridden by game_event.start_time)
             // if none is found we don't modify start date and use the one in game_event
         }
     }

src/server/game/Events/HolidayDateCalculator.cpp

@@ -0,0 +1,582 @@
+/*
+ * This file is part of the AzerothCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "HolidayDateCalculator.h"
+#include "SharedDefines.h"
+#include <cmath>
+
+// Constants for astronomical calculations
+constexpr double PI = 3.14159265358979323846;
+constexpr double DEG_TO_RAD = PI / 180.0;
+
+// Helper: sin/cos in degrees
+inline double sind(double deg) { return std::sin(deg * DEG_TO_RAD); }
+
+// Static holiday rules configuration
+static const std::vector<HolidayRule> HolidayRules = {
+    // Lunar Festival: Chinese New Year - 1 day (event starts day before CNY)
+    { HOLIDAY_LUNAR_FESTIVAL, HolidayCalculationType::LUNAR_NEW_YEAR, 0, 0, 0, -1 },
+
+    // Love is in the Air: First Monday on or after Feb 3
+    { HOLIDAY_LOVE_IS_IN_THE_AIR, HolidayCalculationType::WEEKDAY_ON_OR_AFTER, 2, 3, static_cast<int>(Weekday::MONDAY), 0 },
+
+    // Noblegarden: Day after Easter Sunday (Easter + 1 day)
+    { HOLIDAY_NOBLEGARDEN, HolidayCalculationType::EASTER_OFFSET, 0, 0, 0, 1 },
+
+    // Children's Week: First Monday on or after Apr 25 (Monday closest to May 1)
+    { HOLIDAY_CHILDRENS_WEEK, HolidayCalculationType::WEEKDAY_ON_OR_AFTER, 4, 25, static_cast<int>(Weekday::MONDAY), 0 },
+
+    // Midsummer Fire Festival: Fixed Jun 21
+    { HOLIDAY_FIRE_FESTIVAL, HolidayCalculationType::FIXED_DATE, 6, 21, 0, 0 },
+
+    // Fireworks Spectacular: Fixed Jul 4
+    { HOLIDAY_FIREWORKS_SPECTACULAR, HolidayCalculationType::FIXED_DATE, 7, 4, 0, 0 },
+
+    // Pirates' Day: Fixed Sep 19
+    { HOLIDAY_PIRATES_DAY, HolidayCalculationType::FIXED_DATE, 9, 19, 0, 0 },
+
+    // Brewfest: Oktoberfest rule - first Saturday on/after Sept 15, minus 7 for holidayStage offset
+    { HOLIDAY_BREWFEST, HolidayCalculationType::WEEKDAY_ON_OR_AFTER, 9, 15, static_cast<int>(Weekday::SATURDAY), -7 },
+
+    // Harvest Festival: 2 days before autumn equinox (Sept 20-21)
+    { HOLIDAY_HARVEST_FESTIVAL, HolidayCalculationType::AUTUMN_EQUINOX, 0, 0, 0, -2 },
+
+    // Hallow's End: Fixed Oct 18
+    { HOLIDAY_HALLOWS_END, HolidayCalculationType::FIXED_DATE, 10, 18, 0, 0 },
+
+    // Day of the Dead: Fixed Nov 1
+    { HOLIDAY_DAY_OF_DEAD, HolidayCalculationType::FIXED_DATE, 11, 1, 0, 0 },
+
+    // Pilgrim's Bounty: Sunday before Thanksgiving (4th Thursday - 4 days)
+    { HOLIDAY_PILGRIMS_BOUNTY, HolidayCalculationType::NTH_WEEKDAY, 11, 4, static_cast<int>(Weekday::THURSDAY), -4 },
+
+    // Winter Veil: 6 days before winter solstice (Dec 15-16)
+    { HOLIDAY_FEAST_OF_WINTER_VEIL, HolidayCalculationType::WINTER_SOLSTICE, 0, 0, 0, -6 },
+
+    // Darkmoon Faire: First Sunday of months matching (month % 3 == locationOffset)
+    // Rotates monthly: Mulgore (Jan) -> Terokkar (Feb) -> Elwynn (Mar) -> repeat
+    // rule.month stores the location offset
+    // rule.offset is -2 (building phase starts Friday, 2 days before faire opens on Sunday)
+    { HOLIDAY_DARKMOON_FAIRE_ELWYNN, HolidayCalculationType::DARKMOON_FAIRE, 0, 0, 0, -2 },    // Mar, Jun, Sep, Dec
+    { HOLIDAY_DARKMOON_FAIRE_THUNDER, HolidayCalculationType::DARKMOON_FAIRE, 1, 0, 0, -2 },   // Jan, Apr, Jul, Oct
+    { HOLIDAY_DARKMOON_FAIRE_SHATTRATH, HolidayCalculationType::DARKMOON_FAIRE, 2, 0, 0, -2 }  // Feb, May, Aug, Nov
+};
+
+const std::vector<HolidayRule>& HolidayDateCalculator::GetHolidayRules()
+{
+    return HolidayRules;
+}
+
+std::tm HolidayDateCalculator::CalculateEasterSunday(int year)
+{
+    // Anonymous Gregorian algorithm (Computus)
+    // Reference: https://en.wikipedia.org/wiki/Date_of_Easter#Anonymous_Gregorian_algorithm
+    int const a = year % 19;
+    int const b = year / 100;
+    int const c = year % 100;
+    int const d = b / 4;
+    int const e = b % 4;
+    int const f = (b + 8) / 25;
+    int const g = (b - f + 1) / 3;
+    int const h = (19 * a + b - d - g + 15) % 30;
+    int const i = c / 4;
+    int const k = c % 4;
+    int const l = (32 + 2 * e + 2 * i - h - k) % 7;
+    int const m = (a + 11 * h + 22 * l) / 451;
+    int const month = (h + l - 7 * m + 114) / 31;
+    int const day = ((h + l - 7 * m + 114) % 31) + 1;
+
+    std::tm result = {};
+    result.tm_year = year - 1900;
+    result.tm_mon = month - 1;
+    result.tm_mday = day;
+    mktime(&result); // Normalize and fill in other fields
+
+    return result;
+}
+
+std::tm HolidayDateCalculator::CalculateNthWeekday(int year, int month, Weekday weekday, int n)
+{
+    // Start with first day of the month
+    std::tm date = {};
+    date.tm_year = year - 1900;
+    date.tm_mon = month - 1;
+    date.tm_mday = 1;
+    mktime(&date);
+
+    // Find first occurrence of the target weekday
+    int const daysUntilWeekday = (static_cast<int>(weekday) - date.tm_wday + 7) % 7;
+    date.tm_mday = 1 + daysUntilWeekday;
+
+    // Move to nth occurrence
+    date.tm_mday += (n - 1) * 7;
+
+    mktime(&date); // Normalize (handles month overflow)
+    return date;
+}
+
+std::tm HolidayDateCalculator::CalculateWeekdayOnOrAfter(int year, int month, int day, Weekday weekday)
+{
+    // Start with the specified date
+    std::tm date = {};
+    date.tm_year = year - 1900;
+    date.tm_mon = month - 1;
+    date.tm_mday = day;
+    mktime(&date);
+
+    // Find days until the target weekday (0 if already on that day)
+    int const daysUntilWeekday = (static_cast<int>(weekday) - date.tm_wday + 7) % 7;
+    date.tm_mday += daysUntilWeekday;
+
+    mktime(&date); // Normalize
+    return date;
+}
+
+// ============================================================================
+// LUNAR NEW YEAR CALCULATION
+// Based on Jean Meeus "Astronomical Algorithms" (1991), Chapter 49
+// Reference: https://celestialprogramming.com/moonphases.html
+// Chinese New Year = new moon falling between January 21 and February 20
+// ============================================================================
+
+double HolidayDateCalculator::DateToJulianDay(int year, int month, double day)
+{
+    if (month <= 2)
+    {
+        year -= 1;
+        month += 12;
+    }
+    int const A = year / 100;
+    int const B = 2 - A + (A / 4);
+    return std::floor(365.25 * (year + 4716)) + std::floor(30.6001 * (month + 1)) + day + B - 1524.5;
+}
+
+void HolidayDateCalculator::JulianDayToDate(double jd, int& year, int& month, int& day)
+{
+    jd += 0.5;
+    int const Z = static_cast<int>(jd);
+    int A = Z;
+    if (Z >= 2299161)
+    {
+        int const alpha = static_cast<int>((Z - 1867216.25) / 36524.25);
+        A = Z + 1 + alpha - (alpha / 4);
+    }
+    int const B = A + 1524;
+    int const C = static_cast<int>((B - 122.1) / 365.25);
+    int const D = static_cast<int>(365.25 * C);
+    int const E = static_cast<int>((B - D) / 30.6001);
+
+    day = B - D - static_cast<int>(30.6001 * E);
+    month = (E < 14) ? E - 1 : E - 13;
+    year = (month > 2) ? C - 4716 : C - 4715;
+}
+
+double HolidayDateCalculator::CalculateNewMoon(double k)
+{
+    // Meeus "Astronomical Algorithms" Chapter 49
+    double const T = k / 1236.85;
+    double const T2 = T * T;
+    double const T3 = T2 * T;
+    double const T4 = T3 * T;
+
+    // Mean phase (Eq 49.1)
+    double const JDE = 2451550.09766 + 29.530588861 * k + 0.00015437 * T2
+                     - 0.000000150 * T3 + 0.00000000073 * T4;
+
+    // Eccentricity correction
+    double const E = 1.0 - 0.002516 * T - 0.0000074 * T2;
+    double const E2 = E * E;
+
+    // Sun's mean anomaly (Eq 49.4)
+    double const M = 2.5534 + 29.10535670 * k - 0.0000014 * T2 - 0.00000011 * T3;
+
+    // Moon's mean anomaly (Eq 49.5)
+    double const MPrime = 201.5643 + 385.81693528 * k + 0.0107582 * T2
+                        + 0.00001238 * T3 - 0.000000058 * T4;
+
+    // Moon's argument of latitude (Eq 49.6)
+    double const F = 160.7108 + 390.67050284 * k - 0.0016118 * T2
+                   - 0.00000227 * T3 + 0.000000011 * T4;
+
+    // Longitude of ascending node (Eq 49.7)
+    double const Omega = 124.7746 - 1.56375588 * k + 0.0020672 * T2 + 0.00000215 * T3;
+
+    // New Moon corrections (Table 49.A)
+    double correction =
+        - 0.40720 * sind(MPrime)
+        + 0.17241 * E * sind(M)
+        + 0.01608 * sind(2 * MPrime)
+        + 0.01039 * sind(2 * F)
+        + 0.00739 * E * sind(MPrime - M)
+        - 0.00514 * E * sind(MPrime + M)
+        + 0.00208 * E2 * sind(2 * M)
+        - 0.00111 * sind(MPrime - 2 * F)
+        - 0.00057 * sind(MPrime + 2 * F)
+        + 0.00056 * E * sind(2 * MPrime + M)
+        - 0.00042 * sind(3 * MPrime)
+        + 0.00042 * E * sind(M + 2 * F)
+        + 0.00038 * E * sind(M - 2 * F)
+        - 0.00024 * E * sind(2 * MPrime - M)
+        - 0.00017 * sind(Omega);
+
+    // Additional planetary corrections (Table 49.B)
+    double const A1  = 299.77 +  0.107408 * k - 0.009173 * T2;
+    double const A2  = 251.88 +  0.016321 * k;
+    double const A3  = 251.83 + 26.651886 * k;
+    double const A4  = 349.42 + 36.412478 * k;
+    double const A5  =  84.66 + 18.206239 * k;
+    double const A6  = 141.74 + 53.303771 * k;
+    double const A7  = 207.14 +  2.453732 * k;
+    double const A8  = 154.84 +  7.306860 * k;
+    double const A9  =  34.52 + 27.261239 * k;
+    double const A10 = 207.19 +  0.121824 * k;
+    double const A11 = 291.34 +  1.844379 * k;
+    double const A12 = 161.72 + 24.198154 * k;
+    double const A13 = 239.56 + 25.513099 * k;
+    double const A14 = 331.55 +  3.592518 * k;
+
+    correction += 0.000325 * sind(A1) + 0.000165 * sind(A2) + 0.000164 * sind(A3)
+                + 0.000126 * sind(A4) + 0.000110 * sind(A5) + 0.000062 * sind(A6)
+                + 0.000060 * sind(A7) + 0.000056 * sind(A8) + 0.000047 * sind(A9)
+                + 0.000042 * sind(A10) + 0.000040 * sind(A11) + 0.000037 * sind(A12)
+                + 0.000035 * sind(A13) + 0.000023 * sind(A14);
+
+    return JDE + correction;
+}
+
+std::tm HolidayDateCalculator::CalculateLunarNewYear(int year)
+{
+    // Chinese New Year always falls on the new moon between Jan 21 and Feb 20
+    double const jan21JD = DateToJulianDay(year, 1, 21.0);
+    double const feb21JD = DateToJulianDay(year, 2, 21.0);
+
+    // Approximate lunation number k for January of target year
+    double const approxK = (year - 2000.0) * 12.3685;
+    double const k = std::floor(approxK);
+
+    // Search for the new moon in the valid range
+    for (int i = -2; i <= 2; ++i)
+    {
+        double const nmJDE = CalculateNewMoon(k + i);
+
+        // Convert TT (Terrestrial Time) to UT (approximate DeltaT ~70s for 2020s)
+        double nmJD = nmJDE - 70.0 / 86400.0;
+
+        // Add 8 hours for China Standard Time (UTC+8)
+        nmJD += 8.0 / 24.0;
+
+        if (nmJD >= jan21JD && nmJD < feb21JD)
+        {
+            int cnyYear, cnyMonth, cnyDay;
+            JulianDayToDate(nmJD, cnyYear, cnyMonth, cnyDay);
+
+            std::tm result = {};
+            result.tm_year = cnyYear - 1900;
+            result.tm_mon = cnyMonth - 1;
+            result.tm_mday = cnyDay;
+            mktime(&result);
+            return result;
+        }
+    }
+
+    // Fallback (should never happen for years 2000-2031)
+    std::tm fallback = {};
+    fallback.tm_year = year - 1900;
+    fallback.tm_mon = 0;  // January
+    fallback.tm_mday = 25;
+    mktime(&fallback);
+    return fallback;
+}
+
+// ============================================================================
+// AUTUMN EQUINOX CALCULATION
+// Based on Jean Meeus "Astronomical Algorithms" (1991), Chapter 27
+// Reference: https://en.wikipedia.org/wiki/Equinox#Calculation
+// ============================================================================
+
+std::tm HolidayDateCalculator::CalculateAutumnEquinox(int year)
+{
+    // Meeus algorithm for mean September equinox (Table 27.C)
+    // Valid for years 2000-3000
+    double const Y = (year - 2000.0) / 1000.0;
+    double const Y2 = Y * Y;
+    double const Y3 = Y2 * Y;
+    double const Y4 = Y3 * Y;
+
+    // Mean equinox JDE0 (Eq 27.1 for September equinox after 2000)
+    double const JDE0 = 2451810.21715 + 365242.01767 * Y - 0.11575 * Y2
+                      + 0.00337 * Y3 + 0.00078 * Y4;
+
+    // Periodic terms for correction (Table 27.B)
+    double const T = (JDE0 - 2451545.0) / 36525.0;
+    double const W = 35999.373 * T - 2.47;
+    double const deltaLambda = 1.0 + 0.0334 * std::cos(W * DEG_TO_RAD)
+                             + 0.0007 * std::cos(2.0 * W * DEG_TO_RAD);
+
+    // Simplified correction (sum of periodic terms from Table 27.C)
+    // Using first few significant terms
+    double const S = 485 * std::cos((324.96 + 1934.136 * T) * DEG_TO_RAD)
+                   + 203 * std::cos((337.23 + 32964.467 * T) * DEG_TO_RAD)
+                   + 199 * std::cos((342.08 + 20.186 * T) * DEG_TO_RAD)
+                   + 182 * std::cos((27.85 + 445267.112 * T) * DEG_TO_RAD)
+                   + 156 * std::cos((73.14 + 45036.886 * T) * DEG_TO_RAD)
+                   + 136 * std::cos((171.52 + 22518.443 * T) * DEG_TO_RAD)
+                   + 77 * std::cos((222.54 + 65928.934 * T) * DEG_TO_RAD)
+                   + 74 * std::cos((296.72 + 3034.906 * T) * DEG_TO_RAD)
+                   + 70 * std::cos((243.58 + 9037.513 * T) * DEG_TO_RAD)
+                   + 58 * std::cos((119.81 + 33718.147 * T) * DEG_TO_RAD)
+                   + 52 * std::cos((297.17 + 150.678 * T) * DEG_TO_RAD)
+                   + 50 * std::cos((21.02 + 2281.226 * T) * DEG_TO_RAD);
+
+    double const JDE = JDE0 + (0.00001 * S) / deltaLambda;
+
+    // Convert JDE to calendar date
+    int eqYear;
+    int eqMonth;
+    int eqDay;
+    JulianDayToDate(JDE, eqYear, eqMonth, eqDay);
+
+    std::tm result = {};
+    result.tm_year = eqYear - 1900;
+    result.tm_mon = eqMonth - 1;
+    result.tm_mday = eqDay;
+    mktime(&result);
+    return result;
+}
+
+// ============================================================================
+// WINTER SOLSTICE CALCULATION
+// Based on Jean Meeus "Astronomical Algorithms" (1991), Chapter 27
+// ============================================================================
+
+std::tm HolidayDateCalculator::CalculateWinterSolstice(int year)
+{
+    // Meeus algorithm for mean December solstice (Table 27.C)
+    // Valid for years 2000-3000
+    double const Y = (year - 2000.0) / 1000.0;
+    double const Y2 = Y * Y;
+    double const Y3 = Y2 * Y;
+    double const Y4 = Y3 * Y;
+
+    // Mean solstice JDE0 (Eq 27.1 for December solstice after 2000)
+    double const JDE0 = 2451900.05952 + 365242.74049 * Y - 0.06223 * Y2
+                      - 0.00823 * Y3 + 0.00032 * Y4;
+
+    // Periodic terms for correction (Table 27.B)
+    double const T = (JDE0 - 2451545.0) / 36525.0;
+    double const W = 35999.373 * T - 2.47;
+    double const deltaLambda = 1.0 + 0.0334 * std::cos(W * DEG_TO_RAD)
+                             + 0.0007 * std::cos(2.0 * W * DEG_TO_RAD);
+
+    // Simplified correction (sum of periodic terms from Table 27.C)
+    double const S = 485 * std::cos((324.96 + 1934.136 * T) * DEG_TO_RAD)
+                   + 203 * std::cos((337.23 + 32964.467 * T) * DEG_TO_RAD)
+                   + 199 * std::cos((342.08 + 20.186 * T) * DEG_TO_RAD)
+                   + 182 * std::cos((27.85 + 445267.112 * T) * DEG_TO_RAD)
+                   + 156 * std::cos((73.14 + 45036.886 * T) * DEG_TO_RAD)
+                   + 136 * std::cos((171.52 + 22518.443 * T) * DEG_TO_RAD)
+                   + 77 * std::cos((222.54 + 65928.934 * T) * DEG_TO_RAD)
+                   + 74 * std::cos((296.72 + 3034.906 * T) * DEG_TO_RAD)
+                   + 70 * std::cos((243.58 + 9037.513 * T) * DEG_TO_RAD)
+                   + 58 * std::cos((119.81 + 33718.147 * T) * DEG_TO_RAD)
+                   + 52 * std::cos((297.17 + 150.678 * T) * DEG_TO_RAD)
+                   + 50 * std::cos((21.02 + 2281.226 * T) * DEG_TO_RAD);
+
+    double const JDE = JDE0 + (0.00001 * S) / deltaLambda;
+
+    // Convert JDE to calendar date
+    int solYear;
+    int solMonth;
+    int solDay;
+    JulianDayToDate(JDE, solYear, solMonth, solDay);
+
+    std::tm result = {};
+    result.tm_year = solYear - 1900;
+    result.tm_mon = solMonth - 1;
+    result.tm_mday = solDay;
+    mktime(&result);
+    return result;
+}
+
+std::tm HolidayDateCalculator::CalculateHolidayDate(const HolidayRule& rule, int year)
+{
+    std::tm result = {};
+
+    switch (rule.type)
+    {
+        case HolidayCalculationType::FIXED_DATE:
+        {
+            result.tm_year = year - 1900;
+            result.tm_mon = rule.month - 1;
+            result.tm_mday = rule.day;
+            mktime(&result);
+            break;
+        }
+        case HolidayCalculationType::NTH_WEEKDAY:
+        {
+            result = CalculateNthWeekday(year, rule.month, static_cast<Weekday>(rule.weekday), rule.day);
+            if (rule.offset != 0)
+            {
+                result.tm_mday += rule.offset;
+                mktime(&result); // Normalize
+            }
+            break;
+        }
+        case HolidayCalculationType::EASTER_OFFSET:
+        {
+            result = CalculateEasterSunday(year);
+            result.tm_mday += rule.offset;
+            mktime(&result); // Normalize
+            break;
+        }
+        case HolidayCalculationType::LUNAR_NEW_YEAR:
+        {
+            result = CalculateLunarNewYear(year);
+            if (rule.offset != 0)
+            {
+                result.tm_mday += rule.offset;
+                mktime(&result); // Normalize
+            }
+            break;
+        }
+        case HolidayCalculationType::WEEKDAY_ON_OR_AFTER:
+        {
+            result = CalculateWeekdayOnOrAfter(year, rule.month, rule.day, static_cast<Weekday>(rule.weekday));
+            if (rule.offset != 0)
+            {
+                result.tm_mday += rule.offset;
+                mktime(&result); // Normalize
+            }
+            break;
+        }
+        case HolidayCalculationType::AUTUMN_EQUINOX:
+        {
+            result = CalculateAutumnEquinox(year);
+            if (rule.offset != 0)
+            {
+                result.tm_mday += rule.offset;
+                mktime(&result); // Normalize
+            }
+            break;
+        }
+        case HolidayCalculationType::WINTER_SOLSTICE:
+        {
+            result = CalculateWinterSolstice(year);
+            if (rule.offset != 0)
+            {
+                result.tm_mday += rule.offset;
+                mktime(&result); // Normalize
+            }
+            break;
+        }
+        case HolidayCalculationType::DARKMOON_FAIRE:
+        {
+            // Return first occurrence for the year
+            // rule.month contains the location offset (0, 1, or 2)
+            int const locationOffset = rule.month;
+
+            // Find first month in the year where month % 3 == locationOffset
+            for (int month = 1; month <= 12; ++month)
+            {
+                if (month % 3 == locationOffset)
+                {
+                    result = CalculateNthWeekday(year, month, Weekday::SUNDAY, 1);
+                    break;
+                }
+            }
+            break;
+        }
+    }
+
+    return result;
+}
+
+uint32_t HolidayDateCalculator::PackDate(const std::tm& date)
+{
+    // WoW packed date format (same as ByteBuffer::AppendPackedTime):
+    // bits 24-28: year offset from 2000 (5 bits = 0-31, valid years 2000-2031)
+    // bits 20-23: month (0-indexed)
+    // bits 14-19: day (0-indexed)
+    // bits 11-13: weekday (0=Sunday, 6=Saturday - POSIX tm_wday)
+    // bits 6-10: hour
+    // bits 0-5: minute
+    int const year = date.tm_year + 1900;
+    // Client uses 5-bit year offset from 2000, so years before 2000 clamp to 0.
+    // If client is patched to support earlier years, update this logic.
+    uint32_t const yearOffset = (year < 2000) ? 0 : static_cast<uint32_t>(year - 2000);
+    uint32_t const month = static_cast<uint32_t>(date.tm_mon);         // Already 0-indexed
+    uint32_t const day = static_cast<uint32_t>(date.tm_mday - 1);      // Convert to 0-indexed
+    uint32_t const weekday = static_cast<uint32_t>(date.tm_wday);      // 0=Sunday, 6=Saturday
+
+    return (yearOffset << 24) | (month << 20) | (day << 14) | (weekday << 11);
+}
+
+std::tm HolidayDateCalculator::UnpackDate(uint32_t packed)
+{
+    std::tm result = {};
+    result.tm_year = static_cast<int>(((packed >> 24) & 0x1F) + 2000 - 1900);
+    result.tm_mon = static_cast<int>((packed >> 20) & 0xF);
+    result.tm_mday = static_cast<int>(((packed >> 14) & 0x3F) + 1);
+    result.tm_wday = static_cast<int>((packed >> 11) & 0x7);
+    result.tm_hour = static_cast<int>((packed >> 6) & 0x1F);
+    result.tm_min = static_cast<int>(packed & 0x3F);
+    mktime(&result); // Normalize and fill in tm_yday, tm_isdst
+    return result;
+}
+
+uint32_t HolidayDateCalculator::GetPackedHolidayDate(uint32_t holidayId, int year)
+{
+    for (auto const& rule : HolidayRules)
+    {
+        if (rule.holidayId == holidayId)
+        {
+            std::tm const date = CalculateHolidayDate(rule, year);
+            return PackDate(date);
+        }
+    }
+    return 0; // Holiday not found
+}
+
+std::vector<uint32_t> HolidayDateCalculator::GetDarkmoonFaireDates(int locationOffset, int startYear, int numYears, int dayOffset)
+{
+    std::vector<uint32_t> dates;
+
+    // Darkmoon Faire is first Sunday of months where (month % 3) == locationOffset
+    // locationOffset 0: Mar, Jun, Sep, Dec - Elwynn (Alliance)
+    // locationOffset 1: Jan, Apr, Jul, Oct - Mulgore (Horde)
+    // locationOffset 2: Feb, May, Aug, Nov - Terokkar (Outland)
+
+    for (int year = startYear; year < startYear + numYears && year <= 2030; ++year)
+    {
+        for (int month = 1; month <= 12; ++month)
+        {
+            if (month % 3 == locationOffset)
+            {
+                // Calculate first Sunday of this month, then apply day offset
+                std::tm date = CalculateNthWeekday(year, month, Weekday::SUNDAY, 1);
+                if (dayOffset != 0)
+                {
+                    date.tm_mday += dayOffset;
+                    mktime(&date); // Normalize
+                }
+                dates.push_back(PackDate(date));
+            }
+        }
+    }
+
+    return dates;
+}

src/server/game/Events/HolidayDateCalculator.h

@@ -0,0 +1,112 @@
+/*
+ * This file is part of the AzerothCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ACORE_HOLIDAY_DATE_CALCULATOR_H
+#define ACORE_HOLIDAY_DATE_CALCULATOR_H
+
+#include <cstdint>
+#include <ctime>
+#include <vector>
+
+enum class HolidayCalculationType
+{
+    FIXED_DATE,        // Same month/day every year (e.g., Dec 25)
+    NTH_WEEKDAY,       // Nth weekday of month (e.g., 4th Thursday of Nov)
+    EASTER_OFFSET,     // Days relative to Easter Sunday
+    LUNAR_NEW_YEAR,    // Chinese New Year (new moon between Jan 21 - Feb 20)
+    WEEKDAY_ON_OR_AFTER, // First weekday on or after a date (e.g., first Monday on or after Feb 3)
+    AUTUMN_EQUINOX,    // Days relative to autumn equinox (offset in days)
+    WINTER_SOLSTICE,   // Days relative to winter solstice (offset in days)
+    DARKMOON_FAIRE     // First Sunday of months matching (month % 3 == locationOffset)
+};
+
+enum class Weekday
+{
+    SUNDAY = 0,
+    MONDAY,
+    TUESDAY,
+    WEDNESDAY,
+    THURSDAY,
+    FRIDAY,
+    SATURDAY
+};
+
+struct HolidayRule
+{
+    uint32_t holidayId;
+    HolidayCalculationType type;
+    int month;      // 1-12
+    int day;        // For FIXED_DATE: day of month. For NTH_WEEKDAY: which occurrence (1-5)
+    int weekday;    // For NTH_WEEKDAY: 0=Sunday through 6=Saturday
+    int offset;     // For EASTER_OFFSET: days after Easter (can be negative)
+};
+
+class HolidayDateCalculator
+{
+public:
+    // Calculate Easter Sunday for a given year (Computus algorithm)
+    static std::tm CalculateEasterSunday(int year);
+
+    // Calculate Nth weekday of a month (e.g., 4th Thursday of November)
+    static std::tm CalculateNthWeekday(int year, int month, Weekday weekday, int n);
+
+    // Calculate first weekday on or after a specific date (e.g., first Monday on or after Feb 3)
+    static std::tm CalculateWeekdayOnOrAfter(int year, int month, int day, Weekday weekday);
+
+    // Calculate Chinese New Year (Lunar New Year) using astronomical algorithm
+    // Based on Jean Meeus "Astronomical Algorithms" - finds new moon between Jan 21 - Feb 20
+    static std::tm CalculateLunarNewYear(int year);
+
+    // Calculate Autumn Equinox using astronomical algorithm
+    // Based on Jean Meeus "Astronomical Algorithms" Chapter 27
+    static std::tm CalculateAutumnEquinox(int year);
+
+    // Calculate Winter Solstice using astronomical algorithm
+    // Based on Jean Meeus "Astronomical Algorithms" Chapter 27
+    static std::tm CalculateWinterSolstice(int year);
+
+    // Calculate holiday start date for a given year
+    static std::tm CalculateHolidayDate(const HolidayRule& rule, int year);
+
+    // Convert std::tm to WoW's packed date format
+    static uint32_t PackDate(const std::tm& date);
+
+    // Convert WoW's packed date format to std::tm
+    static std::tm UnpackDate(uint32_t packed);
+
+    // Get all holiday rules
+    static const std::vector<HolidayRule>& GetHolidayRules();
+
+    // Calculate date for a specific holiday ID and year
+    static uint32_t GetPackedHolidayDate(uint32_t holidayId, int year);
+
+    // Calculate Darkmoon Faire dates for a location over a year range
+    // locationOffset: 0=Elwynn (months 3,6,9,12), 1=Mulgore (months 1,4,7,10), 2=Terokkar (months 2,5,8,11)
+    // dayOffset: days to offset from first Sunday (e.g., -2 for Friday start)
+    // Returns packed dates for all occurrences in the year range
+    static std::vector<uint32_t> GetDarkmoonFaireDates(int locationOffset, int startYear, int numYears, int dayOffset = 0);
+
+private:
+    // Julian Date conversions for lunar calculations
+    static double DateToJulianDay(int year, int month, double day);
+    static void JulianDayToDate(double jd, int& year, int& month, int& day);
+
+    // Calculate new moon Julian Date for lunation k (Meeus algorithm)
+    static double CalculateNewMoon(double k);
+};
+
+#endif // ACORE_HOLIDAY_DATE_CALCULATOR_H