Source: Beliefnet
http://www.Beliefnet.com/frameset.asp?boardID=15665&pageloc=/story/25/story_2530_1.html
On May 16, 2003 Beliefnet reported that "Vesak, or Visakha (pronounced way-sak), is a celebration that commemorates the Buddha's birth, enlightenment, and death, and his passing into nirvana. It is named for the month of May and is celebrated on the full moon, when the Buddha's mother is said to have given birth to him in a garden in the Himalayan foothills while en route to her parents' home. According to most Buddhist calendars, he would be 2,547 years old today (May 16)... Vesak is the most important holiday in the Theravada Buddhist countries of Southeast Asia, though its observance varies from culture to culture. In the United States, it has become the occasion for a common celebration that unites different Buddhist traditions and schools, Asian and non-Asian, immigrant and convert, Theravadin and Mahayana (for different schools of Zen, which is a tradition of the Mahayana school of Buddhism, the Buddha’s birthday is celebrated according to a different calendar and falls on April 8; his enlightenment and death are also assigned to different days)."