Visakha Puja: Birth, Awakening, and Death of the Buddha
(Also spelled/pronounced Wesak)The full moon Uposatha day of (typically)the 5th or 6th lunar month on the Buddhist calendar. Also the name of that month of the lunar calendar. In the modern world, Vesak usually takes place in mid to late May, depending on local tradition, lunar observances, and the differences between the lunar calendar and the Gregorian calendar. During leap years, Vesak falls in June
Vesak commemorates the birth, enlightenment, and death/paranibbana of the Buddha- whose life is traditionally held to have begun, ended, and reached its zenith on the same full-moon night in late Spring.
The day of Vesak (also called Visakha Puja) is the mot significant religious holiday in many Buddhist countries, including all of the countries of the Theravada tradition (primarily Southeast Asia), as well as a number of Mahayana nations (particularly Tibet). Vesak is sometimes called Buddha Day, which is also the English name commonly used for Hanamatsuri (Flower Festival), the Japanese observance of the birth of the Buddha (his birth, death and enlightenment are celebrated on separate days in Japan and most Mahayana nations).
Vesak is celebrated in different ways throughout the world. As an uposatha day, it's traditional for lay Buddhists to visit their local temple, to give offerings to the temple or its monks, and to hear sermons on morality and Buddhist teachings from the Sangha. Tradition holds that acts performed on Vesak have a greater impact than those done at other times of the year- the merit reaped from good deeds is greater, but the consequences of negative actions are more severe as well. I'm not aware of a basis for this in the Pali canon (or any other scripture), but in the popular imagination the laws of kamma (karma) are intensified by the holy day of Vesak (the same goes, to a lesser extent, for other Uposatha days).
Not to say everyone spends their time holed up, worried they'll step on a bug; Buddhist festivals are generally joyful affairs, and Vesak is often marked by festive decorations, charitable giving to religious institutions and the poor, and good food. Like many non-Buddhist holidays (Christmas, Dwali, Chaunukah), Vesak is a festival of light, and lanterns (often in the shape of lotus flowers), candles, and electric lights are all used as decorations (many of the latter bought on sale after Christmas!). Images of the Buddha as a child are quite common- many Vesak ceremonies include bathing a statue of the baby Buddha with perfumed water, re-enacting his bathing by friendly devas following his birth. The story of the Buddha's birth may be told, or other Buddhist jataka stories.
Vesak is the most widely celebrated and recognized Buddhist holiday. The World Fellowship of Buddhists has moved to have the full moon day of May celebrated as 'Buddha Day' by Buddhists of all traditions. On December 15th 1999, the United Nations recognized Vesak as an international U.N holiday. Blue Mountain (as well as a few Buddhist-oriented web sites) has even started to offer Vesak e-cards.
Vesak has also played a role in historical events from time to time- outlawing the observance of Vesak was one of the last straws in relations between the pro-Catholic regime of Ngo Dinh Diem and Buddhists in Vietnam, ultimately prompting the spectacular (and often depicted) suicide by self-immolation of Buddhist monk Thich Quang Duc at a Saigon crossroad.