The Buddhist Era (BE) calendar counts years from the death (parinirvana) of the Buddha. In Thailand it is the official calendar, running exactly 543 years ahead of the Gregorian calendar — so 2026 CE is 2569 BE. Cambodia, Laos and Sri Lanka use related Buddhist reckonings.
In Thailand the Buddhist calendar uses the same months and days as the Gregorian calendar, but numbers the years from the Buddhist Era. To convert, simply add 543: 2026 CE = 2569 BE.
The Buddhist Era counts from the parinirvana (passing) of the Buddha, traditionally placed in 543 BC. Different Buddhist countries use slightly different epochs and rules, so the exact BE year can vary by one between, say, Thailand and Sri Lanka.
Thailand's civil dates follow the solar (Gregorian-aligned) Buddhist calendar, but religious festivals such as Makha Bucha, Visakha Bucha and the start of Vassa (Buddhist Lent) follow the lunar calendar and shift each year.
Songkran, the traditional Thai New Year, is celebrated around 13–15 April.