Arbaeen (, "forty"), Chehelom (, , "the fortieth [day]") or Qirkhi, Imamin Qirkhi (, "the fortieth of Imam") is a Shia Muslim religious observance that occurs forty days after the Day of Ashura. It honors the martyrdom of Husayn ibn Ali, the grandson of Muhammad. Imam Husayn ibn Ali and 72 companions were killed by Yazid I's army in the Battle of Karbala in 61 AH (680 CE). Arbaeen or forty days is also the usual length of mourning in many Muslim traditions. Arbaeen is one of the largest pilgrimage gatherings on Earth. Millions of people go to the city of Karbala in Iraq. The pilgrims include Shia, Sunnis, Christians, Yazidi and other faiths.