The Vassa retreat has largely been given up by Mahayana Buddhists, as Mahayana Buddhism has typically flourished in regions without a rainy season, however for Mahayana schools such as Zen and Tibetan Buddhism other forms of retreat are common.
The observation of Vassa is said to originate with the Buddha himself. Gautama Buddha ordered his disciples to observe a pre-existing practice whereby holy men avoided travel for a three month period during the rainy season, in order to avoid damaging crops.
Vassa begins on the first day of the waning moon of the eighth lunar month; the preceding day is Asalha Puja. The focus of celebration by the laity is the first day of Vassa (or Wan Kao Pansa) during which worshippers donate candles and other necessities to temples, in a ceremony which has reached its most extravagant form in the Ubon Ratchathani Candle Festival.
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.