The decision to agree to celebrate Vesak as the Buddha’s birthday was formalized at the first Conference of the World Fellowship of Buddhists held in Sri Lanka in 1950, although festivals at this time in the Buddhist world are a centuries-old tradition. The Resolution that was adopted at the World Conference reads as follows:

“That this Conference of the World Fellowship of Buddhists, while recording its appreciation of the gracious act of His Majesty, the Maharaja of Nepal in making the full-moon day of Vesak a Public Holiday in Nepal, earnestly requests the Heads of Governments of all countries in which large or small number of Buddhists are to be found, to take steps to make the full-moon day in the month of May a Public Holiday in honour of the Buddha, who is universally acclaimed as one of the greatest benefactors of Humanity."

Vesak is an annual holiday observed by practicing Buddhists in many Asian countries like Thailand, Cambodia, Sri Lanka, Malaysia, Myanmar, Singapore, Vietnam, and also Indonesia, Hong Kong, and Taiwan.

In Indian Mahayana Buddhist traditions, the holiday is known by its Sanskrit equivalent, Vaisakha. The word Vesak itself is the Sinhalese language word for the Pali variation, "Vesakha". Vesak is also known as Visaka Bochea in Cambodia, Visakah Puja, Buddha Purnima or Buddha Jayanti in India, Bangladesh and Nepal, Visakha Bucha in Thailand, in Vietnam, Waisak in Indonesia, Vesak (Wesak) in Sri Lanka and Malaysia, in Chinese-speaking countries, and Saga Dawa in Tibet. The equivalent festival in Laos is called Vixakha Bouxa and in Myanmar is called Ka-sone-la-pyae meaning Fullmoon Day of Kasone which is also the second month of the Myanmar Calendar.

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.