The Vassa (Pali: vassa-, Sanskrit: varṣa-, both "rain") is the three-month annual retreat observed by Theravada practitioners. Taking place during the wet season, Vassa lasts for three lunar months, usually from July (the Burmese month of Waso, ဝါဆို) to October (the Burmese month of Thadingyut သီတင်းကျွတ်).
For the duration of Vassa, monastics remain in one place, typically a monastery or temple grounds. In some monasteries, monks dedicate the Vassa to intensive meditation. Some Buddhist lay people choose to observe Vassa by adopting more ascetic practices, such as giving up meat, alcohol, or smoking. While Vassa is sometimes casually called "Buddhist Lent", others object to this terminology. Commonly, the number of years a monk has spent in monastic life is expressed by counting the number of vassas (or rains) since ordination.
Most Mahayana Buddhists do not observe Vassa, though 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.
The Vassa tradition predates the time of Gautama Buddha. It was a long-standing custom for mendicant ascetics in India not to travel during the rainy season as they may unintentionally harm crops, insects or even themselves during their travels.[non-primary source needed] Many Buddhist ascetics live in regions which lack a rainy season. Consequently, there are places where Vassa may not be typically observed.