For Muslims, this week marks one of their most important holidays, Eid al-Adha (September 12), the festival of the sacrifice. The day commemorates Abraham’s willingness to sacrifice his son. While Jews and Christians believe that son was Isaac, in Islamic tradition it was Ishmael. The Eid comes at the height of the hajj (September 9-14) in Mecca, where pilgrims are performing a series of special rituals.