Arbaeen marks 40 days after the martyrdom of Imam Hussein (AS), the grandson of the Prophet Muhammad (PBUH) and third Shia Imam.
Each year, a huge crowd of Shias march toward Karbala, where the holy shrine of Imam Hussein (AS) is located, to perform mourning rites.
In the past few years, around 20 million pilgrims from all over the world, including Iran, took part in the mourning ceremonies in Iraqi holy cities marking Arbaeen.
This year, Arbaeen falls on Sunday, November 20.