Significance
Buddha Purnima (Vesak) is the holiest day for Buddhists, commemorating three great events said to have occurred on the same full-moon day — the birth, enlightenment and mahaparinirvana of Gautam Buddha.
Hindus revere Buddha as an avatar of Vishnu, so the day is widely observed.
Rituals
Devotees visit viharas and temples, offer flowers and lamps, meditate, listen to discourses on the Dhamma, and practise dana (charity) and non-violence. Bodhi trees are honoured and many observe a vegetarian, contemplative day.
How the date is fixed
Buddha Purnima falls on the full-moon day (Purnima) of Vaishakha.