Vesak is an annual holiday observed traditionally by practicing Buddhists in all
countries. The exact date of Vesak varies according to the various
lunar calendars used in different traditions.
In Theravada countries following the Buddhist calendar, it falls on the full moon Uposatha
day (typically the 5th or 6th lunar month). In China it is the fourth
month in the Chinese lunar calendar, coinciding with the first full
moon of that month. The date varies from year to year in the Western
Gregorian calendar but falls in April or May. Sometimes informally
called "Buddha's birthday," it actually encompasses the
birth, enlightenment Nirvana, and passing (Parinirvana) of Gautama
Buddha.