Shia Muslims gather in Iraq’s holy city of Karbala for Arbaeen on Tuesday, August 4.

The annual event marks the end of a 40-day mourning period, commemorating the martyrdom of Imam Hussein, the Prophet Mohammed’s grandson and the third Shia Imam, who was killed in the 7th century.

The pilgrimage to Karbala for Arbaeen has grown into one of the largest religious gatherings in the world with millions participating.