The Purnima or full moon day of the month of Vaisakha [May] is the day that three very important events in the life of the Buddha occurred. On this day Buddhists celebrate the Buddha's  birth, Enlightenment and Pari-Nirvana. It is the most sacred day in the Buddhist calendar.

The picture above here shows the young LNA nuns chanting  at the Vaisakha celebrations, the story of the Buddha's life - his birth, enlightenment and passing away.