Vesākha (Pali; Sanskrit: Vaiśākha, Devanagari: वैशाख, Sinhala: වෙසක් පෝය) or Vesak is a holy day observed traditionally by Buddhists in Sri Lanka, Bangladesh, India, Nepal and the South East Asian countries of Singapore, Vietnam, Thailand, Cambodia, Laos, Malaysia, Burma, the Philippines, and Indonesia. Sometimes informally called "Buddha's Birthday", it actually encompasses the birth, enlightenment (nirvāna), and passing away (Parinirvāna) of Gautama Buddha.

The exact date of Vesākha varies according to the various lunar calendars used in different traditions. In Theravada countries following the Buddhist calendar, it falls on a full moon Uposatha day, typically in the 5th or 6th lunar month. Vesākha Day in China is on the eighth of the fourth month in the Chinese lunar calendar. The date varies from year to year in the Western Gregorian calendar, but usually falls in April or May. In leap years Vesākha is celebrated in June.

The name of the observance is derived from the Pali term vesākha or Sanskrit vaiśākha, which is the name of the lunar month falling on April to May (see Vaisakha). In Mahayana Buddhist traditions, the holiday is known by its Sanskrit name and derived variants of it. Local renditions of the name varies by country, including: