This day, sometimes called "Sangha Day," commemorates the spontaneous assembly of 1,250 arahants in the Buddha's presence. One thousand of the gathered monks had previously achieved Awakening upon hearing the Buddha's delivery of the Fire Sermon; the remaining 250 were followers of the elder monks Ven. Moggallana and Ven. Sariputta. To mark this auspicious gathering, the Buddha delivered the Ovada-Patimokkha Gatha, a summary of the main points of the Dhamma, which the Buddha gave to the assembly before sending them out to proclaim the doctrine.

Description courtsey of www.accesstoinsight.org