Eid al-Adha or Feast of Sacrifice is the most important feast of the Muslim calendar. It comes at the end of the Pilgrimage to Mecca. Eid al-Adha lasts for three days and commemorates Ibraham's (Abraham) willingness to obey God by sacrificing his son. According to the Koran, the holy book of the Muslim religion, Ibrahim was about to sacrifice his son when a voice from heaven stopped him and allowed him to sacrifice a ram instead. The feast re-enacts Ibrahim's obedience by sacrificing a cow or ram. The family eats about a third of the meal and donates the rest to the poor.

http://www.bbc.co.uk/birmingham/faith/2004/01/eid_al_adha.shtml
http://www.touregypt.net/featurestories/eid.htm (Eid in Egypt)