仏滅紀元
The exact date of Vesak is based on Asian lunisolar calendars and is primarily celebrated in Vaisakha, a month of both the Buddhist and Hindu calendars, hence the name Vesak. In Nepal, which is considered the birth-country of Buddha, it is celebrated on the full moon day of the Vaisakha month of the Hindu calendar, and is traditionally called Buddha Purnima, Purnima meaning the full moon day in Sanskrit. In Theravada countries following the Buddhist calendar, it falls on Uposatha Day, the full moon typically in the 5th or 6th lunar month.