For Theravada Buddhists, Uposatha are days marked by full and new moons, and they are observed with intensive practice.
Monks rededicate themselves to meditation and study. Faults are confessed, and the ancient monastic rules are recited.
For laypeople, Uposatha are days for bringing offerings to the temple, where they may also meditate and listen to dharma talks.
In the Visakhuposatha Sutta the Buddha listed eight ways to observe Uposatha. These eight practices are the Five Precepts plus three more -- to eat just one meal, before noon; to abstain from dancing, singing, wearing jewelry and cosmetics, and from otherwise decorating oneself; and to abstain from sleeping in high, soft beds. These three rules apply to monks all the time, but for Uposatha devout laypeople follow these rules also.
Some full-moon Uposatha days are particularly significant. Among these are:
- Magha Puja. "Sangha Day" commemorates the spontaneous assembly of 1,250 enlightened monks to see the Buddha.
- Vesak (or Visakha) Puja. "Buddha Day" commemorates the birth, enlightenment, and passing into Nirvana of the historical Buddha.
- Asalha Puja. "Dharma Day" commemorates the Buddha's first sermon, in which he explained the Four Noble Truths. This month also marks the beginning of the annual three-month Rains Retreat (Vassa, sometimes called "Buddhist Lent"). During this retreat, monks usually remain cloistered in their monasteries.
- Pavarana Day. This is the ending of Vassa.