Eid-ul-Adha (Festival of Sacrifice) is a festival comes about ten weeks after Eid-ul-Fitr, and marks the completion of Hajj (Holy pilgrimage to Mecca). It is the festival of Sacrifice that commemorate the obedience of Hadhrat Ibrahim and his son Hadhrat Ismail  to the commandment of Allah. On the day of Eid those Muslims who can afford to sacrifice an animal are enjoined to do so after Eid Prayer.

For more history and insight into this auspicious event:
Hazrat Khalifa-tul-Masih Vth (ABA)
Hazrat Khalifa-tul-Masih II (RA)
http://www.alislam.org/eid/