Asalha, the Month of July. This day, sometimes called "Dhamma Day," commemorates the Buddha's first discourse, which he gave to the group of five monks with whom he had practiced in the forest for many years.
See The First Discourse on Turning the Wheel of Dhamma, or in Both Pali and English
Upon hearing this discourse, one of the monks ( Ven. Kondañña) gained his first glimpse of Nibbana, and achieved the 1st sainthood(Sotapanna), thus giving birth to the Noble Sangha.
The annual 3 months Rains retreat (vassa) begins the following day.
As monks used to travel around during the Buddha's time. Lay people complained about the monks destroying their crops. Thus 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.
During Vassa, many Buddhist lay people reinvigorate their spiritual training and adopt more practices. In countries such as Thailand, the laity will often take monastic vows for period of Vassa and return to lay life afterwards.
The Kathina ceremony is held at the month after Vassa, during which the laity gathers to make formal offerings of robe cloth and other requisites to the Sangha.