Asalha Puja (known as Asanha Puja or Asarnha Bucha in Thailand, Thai: อาสาฬหบูชา aa-saal boo-chaa) is a Theravada Buddhist festival which typically takes place in July, on the full moon of the eighth lunar month. It commemorates the Buddha’s first sermon in the Deer Park in Benares and the founding of the Buddhist sangha. In Thailand, Asalha Puja is a government holiday.

The sermon in the Deer Park as depicted at Wat Chedi Liem, Thailand

The day is observed by donating offerings to temples and listening to sermons. The following day is known in Thailand as Wan Kao Pansa; it is the first day of vassa, the Theravada rains retreat.

Vassa (from Pāli vasso, Sanskrit varṣaḥ, both "rain" - Burmese: ဝါဆို waso, Khmer: វស្សា or ព្រះវស្សា; Lao: ພັນສາ pʰán săː, sometimes ວັດສາ wāt săː; Thai: พรรษา,pansa or phansaa; ), also called Rains Retreat, is the traditional retreat during the rainy season lasting for three lunar months from July to October. During this time Buddhist monksremain in a single place, generally in their temples. In some monasteries, monks dedicate the Vassa to intensive meditation. During Vassa, many Buddhist lay people reinvigorate their spiritual training and adopt more ascetic practices, such as giving up meat, alcohol, or smoking (Vassa is sometimes known as "Buddhist Lent", though at least one prominent Theravada monk has objected to this usage- see [1]). And in countries such as Thailand, the laity will often take monastic vows for period of Vassa and return to lay life afterwards. Commonly, the number of years a monk has spent in monastic life is expressed by counting the number of Vassas he has observed.

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 - Lao: ວັນເຂົ້າພັນສາ wán pʰán săː) 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.

Vassa is followed by two of the major festivals of the year among Theravada Buddhists, Wan Awk Pansa and Kathina.