Today is Eid al-Adha, the Feast of Sacrifice, the most eminent holiday on the Muslim calendar. It is celebrated on the 10th day of the month of Dul-Hijja, 70 days after the conclusion of the holy month of Ramadan.
Eid al-Adha commemorates Abraham's commitment to obey God.
The Quran tells how one of Abraham's chief trials was to face God's command to sacrifice his only son as a burnt offering. Abraham prepared to submit to his Lord's will. However, just before Abraham killed his son, God revealed that Abraham's sacrifice had already been fulfilled because Abraham showed that his love for God superceded all others and that he would give up his most loved one for the sake of God. Therefore, God spared Ishmael, and instead allowed Abraham to sacrifice a ram.
Directly preceeding Eid al-Adha, millions of Muslims from all around the world gather in the sacred city of Mecca in Saudi Arabia to perform the annual pilgrimage called Hajj. Hajj is mandatory at least once in the life of every Muslim who is financially and physically able to undergo the pilgrimage.