alt=MONK AT VASSA|thumb|Monk at Vassa
Vassa (, , both "rain") is the three-month annual retreat observed by Theravada Buddhists. Taking place during the wet season, Vassa lasts for three lunar months, usually from July (the Burmese month of Waso, ) to October (the Burmese month of Thadingyut, ).
In English, Vassa is also called Rains Retreat. While Vassa is sometimes casually called "Buddhist Lent", others object to this terminology. In some monasteries, monks dedicate the Vassa to intensive meditation. Commonly, the number of years a monk has spent in monastic life is expressed by counting the number of vassas (or rains) since ordination.
Mahayana Buddhists also observe Vassa. Vietnamese Thiền and Korean Seon monastics observe an equivalent retreat of three months of intensive practice in one location, a practice also observed in Tibetan Buddhism.
Vassa begins on the first day of the waning moon of the eighth lunar month, which is the day after Asalha Puja or Asalha Uposatha ("Dhamma day"). It ends on Pavarana, when all monastics come before the sangha and atone for any offense that might have been committed during Vassa.
Vassa is followed by Kathina, a festival in which the laity expresses gratitude to monks. Lay Buddhists bring donations to temples, especially new robes for the monks.
The Vassa tradition predates the time of Gautama Buddha.