Uposatha is the traditional Buddhist observance day, kept on the days of the new, full and quarter moons, so about four times a lunar month, and it gives the Buddhist calendar its underlying rhythm much as the sabbath does in the Abrahamic traditions. The practice is ancient, going back to the Buddha's own direction that the community should gather on these days; it was adapted from an older Indian custom of pausing on the moon-days, and the Buddha is recorded as instructing the monks to make it an occasion for the recitation of their discipline.
The day has two faces, monastic and lay. For the monastic community (sangha), the full-moon and new-moon Uposathas are when the monks assemble to recite the Patimokkha, the code of some 227 rules (in the Theravada Vinaya), and to confess and make amends for any breach, a communal act that maintains the purity and unity of the order. For lay followers, the day is a time to visit the monastery, offer food to the monks, hear teachings and meditate, and many undertake the Eight Precepts for the day, the usual five with additional undertakings such as not eating after noon and abstaining from entertainment and adornment, a temporary and voluntary intensification of practice. Uposatha thus structures the shared life of monks and laity and keeps the discipline and the teaching regularly renewed.