Photos: Arbaeen procession in besieged Shiite towns of Nubul & al Zahra, Syria

Arbaeen procession in besieged Shiite towns of Nubul & al Zahra, Syria.

Arba'een (Arabic: الأربعين‎, "forty"), Chehelom (Persian: چهلم‎‎, the fortieth [day]") is a Shia Muslim religious observance that occurs forty days after the Day of Ashura. It commemorates the martyrdom of Hussain ibn Ali, the grandson of Prophet Muhammad (PBUT), which falls on the 20th day of the month of Safar. Imam Hussain ibn Ali and 72 companions were martyred by tyrant Yazid I's army in the Battle of Karbala in 61 AH (680 CE). Arbaeen is one of the largest pilgrimage gatherings on Earth, in which up to 30 million people go to the city of Karbala in Iraq.

Arbaeen is the world's largest annual gathering, with the number of pilgrims far exceeding the two million visitors who descend on Mecca for the Hajj. The Kumbh Mela, a mass Hindu pilgrimage, attracts more pilgrims but is held only every three years.