Eid al-Adha (Arabic: عيد الأضحى ‘Īd ul-’Aḍḥā) or the Festival of Sacrifice is a religious festival celebrated by Muslims and Druze worldwide in commemoration of the willingness of Ibrahim (Abraham) to sacrifice his son as an act of obedience to God. The devil tempted Ibrahim by saying he should disobey God and spare his son. As Ibrahim was about to sacrifice his son, God intervened and instead provided a lamb as the sacrifice. This is why today all over the world Muslims who have the means to, sacrifice an animal (usually a goat or a sheep), as a reminder of Ibrahim's obedience to God. The meat is then shared out with family, friends (Muslims or non-Muslims), as well as the poor members of the community. (Islam names Ishmael as the son who was to be sacrificed, whereas the Judeo-Christian name Isaac).
Eid al-Adha is one of two Eid festivals celebrated by Muslims, whose basis comes from the Quran. (Muslims in Iran celebrate a third, non-denominational Eid.) Like Eid el-Fitr, Eid al-Adha begins with a short prayer followed by a sermon (khuṭba).
Eid al-Adha annually falls on the 10th day of the month of Dhul Hijja (ذو الحجة) of the lunar Islamic calendar. The festivities last for two to three days or more depending on the country. Eid al-Adha occurs the day after the pilgrims conducting Hajj, the annual pilgrimage to Mecca in Saudi Arabia by Muslims worldwide, descend from Mount Arafat. It happens to be approximately 70 days after the end of the month of Ramadan.
source : Wikipedia