Vassa (from Pāli vasso, Sanskrit varṣaḥ, both "rain" - Burmese: ဝါတွင်း, ; Khmer: វស្សា or ព្រះវស្សា; Lao: ພັນສາ, sometimes ວັດສາ ; Thai: พรรษา, phansaa), also called Rains Retreat, and sometimes called Buddhist Lent in English language publications. (This is anachronistic as the so-called Buddhist Lent was established by the Buddha around 500 years before the birth of Christianity.) Vassa is the three-month annual retreat observed by Theravada practitioners. Taking place during the rainy 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, Bhikkhus remain inside monasteries and 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.
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 pre-dates the time of the historical 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.
Many Buddhist asectics live in regions which don't have rainy seasons and therefore Vassa is not observed worldwide.