The occasion called Arbaeen occurs 40 days after Ashura, the tenth day of the lunar calendar month of Muharram, when the grandson of Prophet Muhammad was martyred at the hands of the tyrant of the time, Yazid I, back in the seventh century.
[caption id="" align="alignnone" width="555"]

The first nine days of Muharram feature mourning processions in anticipation of Ashura. The rituals culminate with Arbaeen, known as the worlds largest annual pilgrimage.
Large numbers of Iranians travel to Iraq each year, walking the distance between the Iraqi holy cities of Najaf and Karbala, which houses the Imams holy shrine, so as to be able to be there by Arbaeen.
[caption id="" align="alignnone" width="555"]

According to the spokesman for Irans Arbaeen ceremony committee, Shahriar Heidari, so far 2.3 million pilgrims have travelled to Iraq to mark Arbaeen this year, including 80,000 Afghan expatriates living in Iran.
The devotees are joined by millions of their fellow Muslims from other countries along the course.
[caption id="" align="alignnone" width="555"]

This year, Iranians mark Arbaeen on Thursday, but the occasion will be commemorated in Karbala on Friday.
Imam Hussein is a highly-venerated figure not only among Shias but also Sunni Muslims. Even some groups of Iraqi Christians join the march to Karbala.