Uposatha and festival calendar

The uposatha and festivals tools say when the uposatha (observance) days and the main Theravāda festivals fall, for four traditions. They work offline.

Every date is computed, not official. Each answer gives the rule it used and is labelled status: "computed". National authorities, Sangha bodies and individual monasteries publish the dates they actually keep, and these can differ, usually by one day. If a date matters, check the list your monastery or government publishes.

What you can ask

Traditions: thai-mahanikaya, thai-dhammayut, myanmar, sri-lanka. Years 1950–2100.

The four reckonings

The traditions do not use one calendar. They differ in how they count months and in which day they call the full moon. All times are local: Thailand UTC+7, Myanmar UTC+6:30, Sri Lanka UTC+5:30.

Thai Mahānikāya: the Thai lunar calendar

The Thai civil lunar calendar, the one printed in Thai diaries and used for public holidays.

• Months alternate 29 days (odd months) and 30 days (even months). Each month has a waxing half of 15 days and a waning half of 14 or 15 days.
• A year is common (354 days), adhikavāra (355 days: month 7 gets a 30th day) or adhikamāsa (384 days: a second month 8, “second Āsāḷha”). Which one is decided by the Suriyayatra rules of the Chulasakarat era: from the year’s horakhun (days since the era began), kammacubala, avoman and the moon’s age (tithi) at New Year.
• Uposatha falls on the 8th and 15th (full moon) of the waxing half and on the 8th and last day (new moon) of the waning half.
• In an adhikamāsa year, Māgha Pūjā moves from month 3 to month 4, Visākha Pūjā from month 6 to month 7, and Āsāḷha Pūjā to the second month 8.

The code is a JavaScript port of splendidmoons by Gambhiro Bhikkhu (MIT), which follows J. C. Eade’s published rules; see its explanation.

Thai Dhammayut: Pakkhagaṇanā

The Dhammayut order counts fortnights with Pakkhagaṇanā (ปักขคณนา), a reckoning designed to stay closer to the real moon than the civil calendar. Fortnights of 14 or 15 days are laid out on a counting board in nested cycles (of 16,168, 1,447, 251, 59 and 15 days) from an epoch of 28 January 1736. Uposatha falls on the 8th and the last day of each fortnight. As a result, some Dhammayut uposatha days differ from the Mahānikāya ones by one day, earlier or later. In 2024–2026 there are 40 such days: on 14 the Pakkhagaṇanā day is one day earlier (for example 8 August 2025, against 9 August), on 26 one day later.

The code is a port of PakDate in pythaidate by Mark Hollow (MIT). Every Dhammayut day also gives mahanikaya_date, the matching Mahānikāya day, so you can see where the two differ. Festival months follow the Thai lunar calendar; each festival takes the Dhammayut full moon nearest the Mahānikāya one. Where the two differ, festivals also gives mahanikaya_date (mahanikaya_start and mahanikaya_end for the Kaṭhina season), names those festivals in a note, and repeats the caveat about Wat Metta’s lists (below).

Myanmar: the Burmese calendar

• The Burmese lunisolar calendar, in the form used since independence (the “third era”, from 1312 ME = 1950 CE).
• The year is fixed by the mean full moon of (second) Waso.
• In watat years a 30-day First Waso is inserted. In big watat years Nayon also gets a 30th day.
• Months alternate 29 and 30 days.
• Uposatha (ဥပုသ်) falls on the 8th, 15th (full moon) and 23rd day and on the last day (new moon) of each month.

The code is a port of ceMmDateTime.js by Yan Naing Aye (MIT), which documents the algorithm in Algorithm, Program and Calculation of Myanmar Calendar. Only the rules of the third era and of the second era (1217–1311 ME, needed for January–March 1950) are ported, which is why the tools start at 1950.

Sri Lanka: poya days

Sri Lankan poya days follow the lunar day (tithi). One tithi is the time it takes the moon to move 12° further ahead of the sun. The full-moon tithi (pūrṇimā) runs from an elongation of 168° to the full moon at 180°.

• Rule. The poya is the day at whose evening (18:00 Sri Lanka time) the full-moon tithi is current. If no 18:00 falls inside the tithi, which happens when it starts after 18:00 and ends before the next 18:00, the poya is the day of the full moon itself.
• Why this rule. No official statement of the rule was found. This one was chosen because it reproduces all 37 poya days in the 2024–2026 gazettes (below). Simpler rules do not: “the calendar day of the full moon” matches only 13 of the 37.
• Months. A month runs from new moon to new moon. It is named by the sidereal sign of the sun at the new moon that opens it, using the Lahiri ayanāṃsa. For example, the sun in Mesha (Aries) gives Vaiśākha, which is Vesak.
• Adhi months. A month in which the sun does not change sign is an adhi (extra) month and takes the next month’s name. An example is Adhi Poson on 30 May 2026.
• Festivals are kept in the regular month, not the adhi one.
• Other phases. New-moon and half-moon days use the same rule with tithis 30, 8 and 23. They are not public holidays and have not been checked against a published list.

The astronomy follows Jean Meeus, Astronomical Algorithms (2nd edition, 1998):

• Moon phases: chapter 49, with all the periodic and additional corrections.
• Moon longitude: chapter 47, using its 26 largest terms. That is accurate to about 0.01°, or about a minute of time at a tithi boundary.
• Sun longitude: chapter 25.
• ΔT: the Espenak–Meeus polynomials.
• Time: everything is computed in UTC and then converted to local time.

Festival rules

Myanmar and Māgha Pūjā. Māgha Pūjā is not among the full-moon public holidays in the official Myanmar lists for 2024–2026, and no published source was found that names the Burmese full moon on which it is kept. The tool returns date: null with that reason rather than guess a month.

The traditions do not always agree on the month. In 2026 Sri Lanka keeps Vesak on 1 May, Myanmar on 30 April and Thailand on 31 May. Thailand has a second Āsāḷha month that year, and Sri Lanka an Adhi Poson.

Texts linked to each festival

Each festival returns the texts associated with it, resolved to the bundled Pāli canon. A test checks that every citation resolves to the intended text.

The canon itself does not say that the birth, awakening and passing fell on the Vesākha full moon, that the first discourse was given on the Āsāḷha full moon, or that the Ovāda-pāṭimokkha was taught on the Māgha full moon. Those dates come from the commentaries and tradition. The tools say so in an association note and do not present them as scripture.

Checked against published calendars, 2024–2026

The reference dates are in tests/fixtures/calendar-reference.json, with the URL of each source. They were copied from the sources, not computed. tests/calendar.test.mjs compares them with the computed dates.

The 17 Dhammayut differences

Wat Metta’s lists match the Pakkhagaṇanā count exactly from January 2024 to April 2025. That includes the 21 days marked with an asterisk (“the Mahānikāya dates fall one day earlier”), where Pakkhagaṇanā and the civil calendar really do differ.

From May 2025 Wat Metta gives the Mahānikāya date on every day where the two reckonings differ. These 17 days are:

• 2025: 11, 19, 26 May; 3 June; 9, 17 August; 7, 15 October; 5, 13 December.
• 2026: 13, 21, 28 August; 5 September; 26 October; 3 November; 24 December.

Its 2026 list says the dates are “for both the Dhammayut Order and the Mahānikāya Order in Thailand”. So the published list stopped following Pakkhagaṇanā that month. Pakkhagaṇanā itself did not change.

The tool keeps the Pakkhagaṇanā day and reports the Mahānikāya day next to it, because the reason for the change was not found in any source. The test lists the 17 days. It fails if any other difference appears, or if any of the 17 is not the Mahānikāya date.

Myanmar reference

The official lists are circulars of the Department of Civil Aviation, the only government lists of Myanmar public holidays for 2024–2026 that could be retrieved. They are plain PDFs. No web-archive copy of them exists, so the fixture records each file’s SHA-256 as retrieved on 2026-11-01; anyone can download the PDF and compare. The 2024 list prints the Waso holiday as “Beginning of Buddhist” (the word after it is cut off); its date is the Waso full moon.

One-day holidays must equal the computed full moon. The Lighting Festival (Thadingyut) and, in 2026, Tazaungmon are given as two- or three-day ranges, so for those the test checks only that the full moon falls inside the range.

An earlier version of the fixture used the Krungsri bank-holiday page for 2026. That page is now behind a bot check and was never archived, so it has been replaced by the government list.

Not checked

• Myanmar new-moon and half-moon days are not public holidays and have no published list here; they come from the same ported calendar as the full moons.
• Sri Lankan new-moon and half-moon days are not public holidays and have not been checked against a published list.
• Myanmar Māgha Pūjā has no date (above).

Limits

• The Sri Lankan rule is fitted to three years of gazettes. In a year where the full-moon tithi begins very close to 18:00, a gazette may differ by a day. The 2026 Poson month is decided by the sun being 0.2° from a sign boundary, which is about four hours of the sun’s motion.
• Myanmar dates are not computed before 1950, because the first era (before 1217 ME) uses other rules.
• Dhammayut monasteries outside Thailand, and some inside it, publish their own lists. Use the tool as a guide and the monastery’s list as the authority.

Code and licences

src/calendar.mjs is plain JavaScript with no data files or network access. It contains ports of three MIT-licensed programs: splendidmoons (© 2023 Gambhiro Bhikkhu), pythaidate (© 2023 Mark Hollow) and ceMmDateTime.js (© 2018 Yan Naing Aye). Their notices are in THIRD_PARTY.md.