Uposatha Calendar

Why the same uposatha can fall on different days

The canon expected it. The Vinaya lays down what to do when two groups of monks count the fortnight a day apart:

The national calendars that fix the days today are not astronomical. Of Sri Lanka’s 13 poya days in 2026, 8 fall the day before the astronomical full moon in Sri Lankan time, and of the 49 Thai wan phra listed for 2026, 21 fall a day before or after the matching moon phase in Thai time, by our comparison with the US Naval Observatory. Access to Insight puts the general point plainly:

That is why the calendar above computes nothing for Sri Lanka, Thailand or Myanmar: it shows each country’s own published list, and gives the astronomical moon separately, never as an observance day. If your temple keeps a different day, keep your temple’s.

The days in the texts

The days are named in the Buddha’s advice to the lay follower Dhammika on keeping the eight-factored observance:

The same days come in a verse spoken by the god Sakka in AN 3.37, which the Buddha says was “poorly sung by Sakka”, because Sakka is not free of greed, hate and delusion; only a perfected monk can rightly say it. What the “special displays” (pāṭihāriyapakkha) are, the discourses do not say; the commentaries, as Ñāṇavara reports them, give several different definitions. For monastics the Vinaya is narrower: the code is recited once a fortnight.

Elsewhere the chapter says:

The eighth is a day for gathering and teaching, as the same chapter has it:

How many of the days a lay person keeps is a matter of circumstance; Bhikkhu Khantipalo describes the custom:

For what the day asks, see uposatha and the eight precepts.

Sri Lanka: poya days, and Vesak 2026

Every full moon is a poya day and a public holiday in Sri Lanka, each with its own name, set out a year ahead in a government gazette under the Holidays Act. In a year with an extra lunar month there are thirteen, and 2026 was one: the gazette named 1 May the Vesak Full Moon Poya Day and 30 May the Adhi Poson Full Moon Poya Day. After the chief prelates of the Sangha’s chapters asked for the change, the State Vesak was held on 30 May instead; the Department of Buddhist Affairs lists “May 30: Vesak Full Moon Poya Day”, and a later gazette, No. 2485/14, moved the holiday after Vesak to 31 May. The calendar shows the gazette’s names with that note. The gazette for 2027 has twelve poya days, Vesak on 19 May.

Thailand: wan phra, and two orders

The Royal Institute Dictionary defines วันพระ as four days a month: the 8th and 15th of the waxing and of the waning moon, or the waning 14th in a short month. We found no list of 2026’s days from the issuing body; the list in the calendar is the one published by the public broadcaster Thai PBS, checked against a calendar image posted by the Government Public Relations Department’s Region 2 office (credited to Dhammanavawang), which agrees on every day but one it misprints. Thailand’s two monastic orders can differ. King Mongkut found that the standard Thai calendar’s moon phases were “sometimes displaced by up to two days relative to the true Moon” and devised his own fortnight reckoning, which, Gislén and Eade report, “is only used by Dhammayuttika Nikaya”. A Dhammayut temple may therefore keep some days on a different date. For 2027, we have found only the three Bucha days, in a Bank of Thailand notice.

Myanmar, Cambodia and Laos

Myanmar’s government circular lists only the full-moon days that are public holidays. Its calendar, like Thailand’s, is reckoned, not observed: a 1901 study by A. M. B. Irwin notes that the calendar’s new moon is “frequently in advance of the real new moon”. Cambodia’s and Laos’s calendars are, in Gislén and Eade’s words, “virtually the same” as Thailand’s. We have found no published uposatha list for these three countries, so the calendar does not invent one.

The Chinese six and ten fasting days

Chinese Buddhist sources, among them the Sutra of the Four Heavenly Kings (T590) and the Sarvāstivāda Vinaya (T1435), name six fasting days in the lunar month: the 8th, 14th, 15th, 23rd, 29th and 30th; the Dà zhìdù lùn (T1509) speaks of keeping the eight precepts on them for “a day and a night” (一日一夜, our gloss). The Kṣitigarbha Sūtra (T412) names ten: the 1st, 8th, 14th, 15th, 18th, 23rd, 24th, 28th, 29th and 30th. None of these texts says what to do in a 29-day month. Later precept manuals do: a lay precept manual compiled by the Ming monk Zhixu, in its later enlarged edition, says 若月小者，取二十八日足之 (our gloss: if the month is short, take the 28th to make up the six), and the modern teacher Yanpei says the same. One Qing manual starts the last three of the ten days on the 27th. The calendar follows these rules and shows the lunar day beside each date.

It works out the lunar months itself. In the Hong Kong Observatory’s words, “Months in the Agricultural Calendar start with a new moon”, reckoned in China’s time zone; the calendar finds each new moon and counts the days from there, and its month starts match every one in the Observatory’s tables from 2024 to 2035.

In Japan the same six days are the rokusainichi (六斎日): the Daijisen dictionary gives the 8th, 14th, 15th, 23rd, 29th and 30th of each month, when lay people keep the eight precepts. Korea’s Encyclopedia of Korean Culture lists the same six days (육재일) among the days of abstinence Korean Buddhists have long kept. We could not confirm how widely, or by which calendar, they are kept in either country today, so the calendar describes them and does not compute them.

Tibetan practice days

In Tibetan practice the days fall by the Tibetan calendar, which is calculated, not observed. FPMT, following Lama Zopa Rinpoche, recommends the eight Mahāyāna precepts on “Days 8, 15 (full moon) and 30 (new Moon) of every Tibetan month”, and in the tantras, a Lotsawa House translation explains, the tenth day of the waxing moon and the twenty-fifth are “special days”. The Tibetan calendar skips some dates and doubles others, and its two main versions, Phugpa and Tsurphu, differ, in Svante Janson’s count, by “about one day in 20”. The calendar does not compute these; follow your lineage’s almanac.

How the dates here are made

Official lists are copied from each publisher and checked date by date against the published document. The moon is computed with Jean Meeus’s method for the phases of the moon, and every phase from 2025 to 2027 agrees with the US Naval Observatory to within a minute. The Chinese days come from those new moons and the rules above. A phase close to midnight is flagged, because a minute decides the day.

Related: the Buddhist calendar, Vesak, and the Buddhist Era converter. See all our free tools.