Buddha's Birthday or Buddha Day (also known as Buddha Jayanti, Buddha Purnima, and Buddha Pournami) is a primarily Buddhist festival that is celebrated in most of South, Southeast and East Asia, commemorating the birth of the prince Siddhartha Gautama, who became the Gautama Buddha and founded Buddhism. According to Buddhist tradition and archaeologists, Gautama Buddha, c. 623 BCE, was born at Lumbini in Nepal. Buddha's mother was Queen Maya Devi, who delivered the Buddha while undertaking a journey to her native home, and his father was King Śuddhodana. The Mayadevi Temple, its gardens, and an Ashoka Pillar dating from 249 BCE mark the Buddha's birthplace at Lumbini.

The exact year of Buddha's birthday is based on the Sri Lankan convention, while several Asian lunisolar calendars ascribe to different lunar days. The date for the celebration of Buddha's birthday therefore varies from year to year in the Western Gregorian calendar, but it is usually celebrated in either April or May. During leap years, the birthday may be celebrated in June.

In South and Southeast Asia, the Buddha's birth is celebrated as part of Vesak, a festival that also celebrates the Buddha's enlightenment (on the day of the full moon, hence Sanskrit: पूर्णिमा pūrṇimā) and his mahaparinirvana. In Tibetan Buddhism, Buddha's birth (7th day of the 4th Month) is celebrated separately from Saga Dawa Duchen, the annual festival celebrating his enlightenment and mahaparinirvana (15th Day of the 4th Month). In East Asia, Vietnam and the Philippines, the enlightenment and death of the Buddha are observed as separate holidays.

The exact date of Buddha's Birthday is based on the Asian lunisolar calendars and is primarily celebrated in Vaisakha month of the Buddhist calendar and the Vikram Samvat Hindu calendar. This is the etymology behind the term Vesak. In modern-day India and Nepal, where the historical Buddha lived, it is celebrated on the full moon day of the Vaisakha month of the Buddhist calendar. In Theravada countries following the Buddhist calendar, it falls on the full moon, Uposatha day, typically in the 5th or 6th lunar month. In China, Korea, Vietnam and the Philippines, it is celebrated on the eighth day 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 it may be celebrated in June. In Tibet, it falls on the 7th day of the fourth month of the Tibetan calendar.

In South Asian and Southeast Asian countries (except Vietnam and the Philippines) as well as Mongolia, Buddha's birthday is celebrated on the full moon day of the Vaisakha month of the Buddhist calendar and the Hindu calendar, which usually falls in April or May of the Western Gregorian calendar. The festival is known as Buddha Purnima, as Purnima means full moon day in Sanskrit. It is also called Buddha Jayanti, with Jayanti meaning birthday in Sanskrit.

The corresponding Western Gregorian calendar dates varies from year to year:

In many East Asian countries as well as Vietnam and the Philippines, Buddha's Birth is celebrated on the 8th day of the 4th month in the Chinese lunar calendar (in Japan since 1873 on April 8 of the Gregorian calendar), and the day is an official holiday in Hong Kong, Macau and South Korea. The date falls from the end of April to the end of May according to the Gregorian calendar.

The primarily solar Gregorian calendar date varies from year to year, and in the year 2021, it was on May 19.