Asalha Puja commemorates the first sermon of the Buddha, preached at the deer park at Sarnath near Varanasi to his five former companions, in which he set out the Four Noble Truths and the Middle Way; the discourse is called the Dhammacakkappavattana Sutta, the "setting in motion of the wheel of the Dhamma", and this event is remembered as the beginning of the Buddha's teaching and of the Sangha, the community. It is kept on the full moon of the month of Asadha (June-July), and is also called Dhamma Day.
The day carries a special weight because the sermon completed the founding of Buddhism as a taught path: with the awakening the Buddha had realised the truth, but with the first sermon he began to make it available, and tradition holds that the first of his hearers attained insight on the spot, so that on this day the "Three Jewels", Buddha, Dhamma and Sangha, were for the first time all present in the world. Asalha Puja is a major observance in the Theravada countries, Thailand, Sri Lanka, Myanmar, Laos and Cambodia, kept with temple visits, offerings, teaching and the circumambulation of shrines. It falls immediately before the beginning of Vassa, the three-month rains retreat, which opens the next day, so that the commemoration of the teaching's beginning also opens the monastic year's most disciplined season.