Ever since the fall of Iraqi dictator Saddam Hussein in 2003, Shiites have begun once again openly marking the annual mourning holiday of Arbaeen, with the number of people taking part in the ceremony surging each year.

Arbaeen, described as the world's largest annual pilgrimage, is a ritual that occurs 40 days after the day the third Shiite Imam Hussein bin Ali was slain by forces loyal to the second Umayyad caliph Yazid in A.D. 680. Hussein, who was the grandson of Prophet Muhammad, is highly cherished and commemorated among Shiites for his battle against the thousands of troops dispatched by Yazid to confront him and his less than 100 companions.

Each year, a few days before Arbaeen, Shiites from all over the world travel to Iraq to walk to the holy shrine of Hussein in the holy city of Karbala. Of note, followers of all branches of Islam venerate Hussein, and even some groups of Christians in Iraq regularly join the march to Karbala.

Last year, 27 million people reportedly took part in the ceremony from various countries, such as Turkey and Tajikistan. Iran is one of the countries that sends the most visitors to the ceremony, with more than 1.5 million pilgrims traveling to Iraq.