The day Buddha was born, the day Buddha attained enlightenment and the day Buddha entered nirvana.
Observances: Meditation, observing the eight precepts, bathing the Buddha
“Vesak”, the Day of the Full Moon in the month of May, is the most sacred day to millions of Buddhists around the world. It was on the Day of Vesak two and a half millennia ago, in the year 623 B.C., that the Buddha was born. It was also on the Day of Vesak that the Buddha attained enlightenment, and it was on the Day of Vesak that the Buddha in his eightieth year passed away.
The General Assembly, by its resolution 54/115 of 1999, recognized internationally the Day of Vesak to acknowledge the contribution that Buddhism, one of the oldest religions in the world, has made for over two and a half millennia and continues to make to the spirituality of humanity. This day is commemorated annually at the UN Headquarters and other UN offices, in consultation with the relevant UN offices and with permanent missions, which also wish to be consulted. Source: United Nations