(or Wesak or Vaisakha) is the Buddhist holiday commemorating at once the birth, enlightenment, and death of the Siddharta Gautama. The name comes from the second month of the Indian calendar, and the holiday is celebrated — according to astrological calculations — on the first full moon of Taurus.
This year there are two full moons during the month of May, so that there is the unusual situation of some Buddhists celebrating tomorrow, May 1, and others celebrating on May 31.
Interestingly, in the Zen and Shin traditions, the Buddha’s enlightenment is celebrated not in May, but in December with the holiday known Rohatsu or Bodhi Day. The Japanese celebrate the Buddha’s birthday in April, and in the Mahayana tradition generally the Buddha’s death — or parinirvana — is celebrated in February.