Arbaʽein, Chehellom is a Shiite religious observance that occurs forty days after the Day of Ashura. It commemorates the martyrdom of Al-Husayn ibn Ali, the grandson of Muhammad, who was martyred on the 10th day of the month of Muharram. Al-Husayn ibn Ali and 71 of his companions were martyred by Ubayd Allah ibn Ziyad’s army in the Battle of Karbala in 61 AH (680 CE).
Arbaʽein or forty days is also the usual length of mourning after the death of a family member or loved one in many Muslim traditions. Arbaʽein is the largest annual pilgrimage in the world , in which up to 45 million people go to the city of Karbala in Iraq.