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:

Gautama Buddha or Siddhārtha Gautama Buddha (Sanskrit: सिद्धार्थ गौतम बुद्ध; Pali: Siddhattha Gotama) was a spiritual teacher from the Indian subcontinent, on whose teachings Buddhism was founded. The word Buddha is a title for the first awakened being in an era. In most Buddhist traditions, Siddhartha Gautama is regarded as the Supreme Buddha (P. sammāsambuddha, S. samyaksaṃbuddha) of our age, "Buddha" meaning "awakened one" or "the enlightened one." Gautama Buddha may also be referred to as Śākyamuni (Sanskrit: शाक्यमुनि "Sage of the Śākyas"). The Buddha found a Middle Way that ameliorated the extreme asceticism found in the Sramana religions.

The time of Gautama's birth and death are uncertain: most early 20th-century historians dated his lifetime as c. 563 BCE to 483 BCE, but more recent opinion dates his death to between 486 and 483 BCE or, according to some, between 411 and 400 BCE. UNESCO lists Lumbini, Nepal, as a world heritage site and birthplace of Gautama Buddha. There are also claims about birth place of Gautama Buddha to be Kapileswara, Orissa or Kapilavastu at Piprahwa, Uttar Pradesh He later taught throughout regions of eastern India such as Magadha and Kośala.