Calendar#
Reference of major calendar systems, religious / cultural observances, and the operator-relevant timing considerations (business days, week start conventions, working week, observance dates that move year-to-year).
For time zones, see Time Zones. For ISO 8601 / strftime formatting, see strftime.
Calendar systems#
System |
Notes |
|---|---|
Gregorian (CE) |
the modern civil default; introduced 1582; in worldwide civil use. |
Julian |
older; Eastern Orthodox liturgical use; lags Gregorian by 13 days. |
Hijri / Islamic |
lunar; AH year count from 622 CE; ~354-day year so dates drift in Gregorian by ~11 days/year. |
Hebrew |
lunisolar; AM year count; major festivals tied to it. |
Hindu (multiple) |
Vikram Samvat (~57 BCE), Shaka Samvat (~78 CE); regional variants. |
Buddhist |
BE year count; SE Asia + Sri Lanka; varies. |
Persian (Solar Hijr |
|
Chinese / Lunar |
lunisolar; spring festival timing. |
Coptic |
Egyptian Orthodox. |
Ethiopian |
~7-8 years behind Gregorian; 13-month structure. |
Bahá’í |
Badí calendar. |
Mayan Long Count |
historical (Mesoamerica). |
ISO 8601 conventions#
Form |
Notes |
|---|---|
Date |
|
Time |
|
Datetime |
|
Week |
|
Ordinal date |
|
Duration |
|
Interval |
|
Time zone |
|
Week conventions#
Convention |
Notes |
|---|---|
ISO 8601 week start |
Monday (1) … Sunday (7). |
US convention |
Sunday (1) … Saturday (7). |
Sabbath |
SAT (Jewish, 7th-day Adventist), FRI (Muslim Friday prayer + half-day or full-day off in Gulf), SUN (Christian). |
Working week (most) |
Mon-Fri (5). |
Working week (Gulf) |
Sun-Thu (Saudi 2013+ shift; UAE 2022 shift to Mon-Fri half; Israel: Sun-Thu). |
Major Christian dates#
Holiday |
Notes |
|---|---|
Christmas Eve / Day |
Dec 24 / 25 (Western); Jan 6 / 7 (Orthodox). |
Epiphany |
Jan 6 (Western); Jan 19 (Orthodox). |
Lent |
begins Ash Wed 46 days before Easter. |
Palm Sunday |
Sun before Easter. |
Maundy Thurs |
Thu before Easter. |
Good Friday |
Fri before Easter; widely observed. |
Easter Sunday |
Western: 1st Sun after the paschal full moon (after vernal equinox). Orthodox uses Julian-based reckoning. |
Pentecost |
50 days after Easter. |
Corpus Christi |
9th day after Pentecost. |
All Saints / All So uls |
Nov 1 / Nov 2. |
Advent |
4 Sundays before Christmas. |
Major Jewish dates#
Holiday |
Notes |
|---|---|
Rosh Hashanah |
Jewish new year; Tishrei 1-2 (Sep / early Oct). |
Yom Kippur |
Day of Atonement; Tishrei 10. |
Sukkot |
Tishrei 15-21. |
Hanukkah |
Kislev 25 - Tevet 2 (8 days; late Nov / Dec). |
Tu BiShvat |
Shevat 15. |
Purim |
Adar 14 (Adar II in leap years). |
Pesach (Passover) |
Nisan 15-22. |
Shavuot |
Sivan 6-7. |
Tisha B’Av |
Av 9. |
Sabbath |
Friday eve to Saturday eve weekly. |
Major Islamic dates#
Holiday |
Notes |
|---|---|
Ramadan |
month of fasting; 9th lunar month; ~30 days. |
Laylat al-Qadr |
27th of Ramadan (commonly). |
Eid al-Fitr |
1 Shawwal (end of Ramadan). |
Eid al-Adha |
10 Dhu al-Hijjah (end of Hajj). |
Hajj |
8-12 Dhu al-Hijjah (annual pilgrimage). |
Islamic New Year |
1 Muharram. |
Ashura |
10 Muharram. |
Mawlid |
12 Rabi al-Awwal (commemoration of Muhammad’s birth). |
Lailat al-Miraj |
27 Rajab. |
Friday prayers |
every Friday at noon. |
Note: dates above shift ~11 days earlier each Gregorian year due to the lunar calendar; Saudi Arabia / UAE government use Hijri for some official purposes.
Major Hindu dates#
Holiday |
Notes |
|---|---|
Diwali |
5-day festival of lights; Oct/Nov. |
Holi |
spring festival of colors; Mar. |
Navratri |
9 nights; twice yearly (Sharad in Sep/Oct, Chaitra in Mar/Apr). |
Dussehra |
Vijayadashami; end of Navratri. |
Janmashtami |
Krishna’s birth; Aug/Sep. |
Ganesh Chaturthi |
Aug/Sep. |
Maha Shivratri |
Feb/Mar. |
Raksha Bandhan |
Aug. |
Onam |
Kerala harvest; Aug/Sep. |
Pongal |
Tamil harvest; mid-Jan. |
Makar Sankranti |
mid-Jan; varies regionally. |
Ram Navami |
Mar/Apr. |
Major Buddhist dates#
Holiday |
Notes |
|---|---|
Vesak / Buddha Day |
full moon of May; the Buddha’s birth, enlightenment, parinirvana. |
Asalha Puja |
full moon of July. |
Magha Puja |
full moon of February/March. |
Bodhi Day |
8 Dec (Mahayana). |
Songkran |
Thai/Lao/Cambodian/Burmese new year; mid-April. |
Losar |
Tibetan new year; Feb. |
Chinese / East Asian#
Holiday |
Notes |
|---|---|
Chinese New Year / Spring Festival |
late Jan or early Feb; ~15-day Spring Festival. |
Lantern Festival |
15th day of 1st lunar month. |
Qingming (Tomb Swee |
|
Dragon Boat |
5th day of 5th lunar month. |
Mid-Autumn |
15th day of 8th lunar month. |
National Day (PRC) |
Oct 1 + golden week. |
National Day (ROC) |
Oct 10. |
Tết (Vietnam) |
lunar new year. |
Seollal (Korea) |
lunar new year. |
Chuseok (Korea) |
mid-autumn. |
Oshogatsu (Japan) |
Jan 1-3 + Golden Week (late Apr / early May) + Obon (mid-Aug). |
US federal holidays (10)#
Holiday |
Date |
|---|---|
New Year’s Day |
Jan 1. |
MLK Day |
3rd Monday in Jan. |
Washington’s BD |
3rd Monday in Feb. |
Memorial Day |
last Monday in May. |
Juneteenth |
Jun 19 (since 2021). |
Independence Day |
Jul 4. |
Labor Day |
1st Monday in Sep. |
Columbus Day |
2nd Monday in Oct. |
Veterans Day |
Nov 11. |
Thanksgiving |
4th Thursday in Nov. |
Christmas Day |
Dec 25. |
UK / EU public holidays#
Holiday |
Notes |
|---|---|
New Year’s Day |
Jan 1; observed across EU + UK. |
Good Friday |
Easter-Friday; UK + most EU. |
Easter Monday |
UK + most EU; not Portugal / Spain in some years. |
Labour Day |
May 1; almost all of EU. |
Christmas Day + |
Dec 25; widely observed. |
Boxing Day |
Dec 26 (UK). |
Bank Holidays |
UK: 1st + last Mon of May, last Mon of Aug. |
Constitution / Nat. |
EU country-specific. |
National Day (FR) |
Bastille Day; Jul 14. |
Unification Day (DE |
) Oct 3. |
Liberation Day (NL) |
May 5. |
Operator notes#
Hijri date computation, multiple methods (umm-al-qura, observation-based); use the official Saudi calendar for authoritative Hajj / Eid dates.
Easter computation, “computus” algorithm; differs Western (Gregorian) vs Eastern (Julian-derived).
Chinese New Year, moves between Jan 21 and Feb 20; Lunar new years across East Asia all align.
Working week, check before scheduling: Israel Sun-Thu; Saudi (since 2013) Sun-Thu; UAE (since 2022) Mon-Fri half; Iran Sat-Wed; Maldives Sun-Thu.
Multi-jurisdiction calendars, pair with locale + region for accurate working-day computation; libraries like python-holidays, workalendar, business-cal handle the per-country logic.
Religious observance scheduling, Ramadan working hours often shorten; Yom Kippur is a complete shutdown in Israel; Diwali / CNY dampen activity in IN / CN respectively for ~1 week.