How the moon phases are calculated
The phases are the moments when the moon’s ecliptic longitude is 0, 90, 180 and 270 degrees from the sun’s. The tool uses the method in chapter 49 of Jean Meeus’s Astronomical Algorithms: a mean phase from the lunation count, about 25 periodic terms for the moon’s and sun’s orbits, and 14 small planetary corrections. The result is converted from Terrestrial Time to Universal Time with the observed value of Delta T, then to your time zone by your browser. Everything runs on your device; nothing is downloaded.
Why the date depends on your time zone
A full moon is a single instant worldwide. If it happens at 04:12 UTC, that is still the evening before in the Americas and already the same morning in Asia, so the calendar date differs by place. This is one reason religious and civil calendars that follow the moon can differ by a day between countries. Type the time zone of the place you care about to see its local dates, for example Asia/Colombo for Sri Lanka or Asia/Bangkok for Thailand.
Buddhist observance days, shown by tradition
Theravada Buddhists keep Uposatha on full and new moon days, and the major festivals fall on full moons, but the dates come from traditional lunar calendars that differ by country and sometimes add a month. In 2026, for example, Sri Lanka added an extra Poya month, Adhi Poson. So the page never presents one country’s date as the date for everyone. Sri Lankan Poya days come from the Central Bank of Sri Lanka holiday list, which follows the government gazette; Thai dates come from the Bank of Thailand’s notice; the Chinese-calendar Buddha’s Birthday is calculated and matches the Hong Kong government lists. Where no official list is included for a year, the page says so: check with your temple.
Vesak in different traditions
Vesak, also called Buddha Day or Visakha Puja, marks the Buddha’s birth, awakening and passing away for many Theravada Buddhists, while East Asian traditions often mark the Buddha’s Birthday on the 8th day of the 4th lunar month. The United Nations describes its Day of Vesak as the day of the full moon in May. In 2027 Sri Lanka’s Vesak Poya is 19 May, Thailand’s Visakha Bucha is 20 May and the Chinese-calendar Buddha’s Birthday is 13 May. Each appears in its own labelled row.
How the dates were checked
We compared every phase in 1901, 1950, 1999, 2025, 2026, 2027, 2049 and 2099, almost 400 in all, with the US Naval Observatory’s published times: every phase fell on the same date and the largest difference was one minute. The official Poya and Thai dates were checked to fall on or next to the astronomical full moon in Colombo and Bangkok. Sources were opened on 16 October 2026. This is general calendar information, not religious guidance.