Sangha Day - Magha Puja
This festival is also known as Fourfold Assembly or Magha Puja Day.
Sangha Day is the second most important Buddhist festival. It is a celebration in honour of the Sangha, or the Buddhist community. For some Buddhists Sangha refers only to monks and nuns. It is a chance for people to reaffirm their commitment to Buddhist practices and traditions.

Sangha Day commemorates the spontaneous gathering of 1,250 enlightened monks (arahants) to hear the Buddha preach at Veluvana Vihara.
At this gathering, the Buddha gave his first sermon, or recitation of the Patimokkha (the rules and regulations of the monastic order).
Sangha is the term used for the Buddhist spiritual community. On Sangha Day Buddhists celebrate both the ideal of creating a spiritual community, and also the actual spiritual community which they are trying to create.
The Sangha is precious in Buddhism as without those in the community to look up to or share aspirations with, the spiritual life would be very challenging.
Sangha Day is a traditional time for exchange of gifts; it has become a prominent festival among Western Buddhists even though it is less well known in the East.
Celebrations vary, but can include chanting, meditation, the lighting of oil lamps, and the reaffirmation of people's commitment to Buddhist practice.

Source from http://www.bbc.co.uk/religion