What is Eid ul-Adha?
Eid ul-Adha is celebrated on the 10th of Dhul Ḥijjah, during the Hajj season. It commemorates the willingness of Ibrāhīm (عليه السلام) to sacrifice his son Ismāʿīl (عليه السلام) in obedience to Allah. Allah replaced the sacrifice with a ram, and this act of submission is remembered every year through the Qurbānī (animal sacrifice).
It is the greater of the two Eids and is celebrated over 3 days (10th, 11th, 12th Dhul Ḥijjah — some scholars include the 13th).