Vassa (Rain Retreat or Winter Retreat in some western countries) is a three-month rainy (or winter) season intensive meditation retreat, and its conclusion are two of the major festivals of the year among Theravada Buddhists. The retreat has largely been given up by Mahayana Buddhists. It is an accepted practice in countries such as Thailand for a layman to take monastic vows for the vassa period and then to return to lay life. Commonly, the number of years a monk has spent in monastic life is expressed by counting up the number of vassas he has observed.

The end of vassa is marked by joyous celebration. The following month, the kathina ceremony is held, during which the laity gathers to make formal offerings of robe cloth and other requisites to the Sangha.