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. Upon hearing this discourse, one of the monks ( Ven. Kondañña) gained his first glimpse of Nibbana, thus giving birth to the Noble Sangha. The annual Rains retreat (vassa) begins the following day. Description courtsey of www.accesstoinsight.org