It is also known as Arba'een, or Chehlom, is one of the largest pilgrimage gatherings on Earth, in which over 15 million people go to the city of Karbala in Iraq

Arba'een (Arabic: "forty") or Chehelom (Persian: "the fortieth [day]"), is a Shia Muslim religious observance that occurs 40 days after the Day of Ashura. It commemorates the martyrdom of Hussein bin Ali, the grandson of the Prophet Muhammad (sw) which falls on the 20th day of the month of Safar. Imam Hussain (as) and 71 supporters martyred in the Battle of Karbala in the year 61 AH (680 CE), killed by Yazid (la) army. Arba'een is also observed 40 days after the death of a family member or loved one.

The occasion reminds the faithful of the core message behind Hussain's (as) martyrdom: establishing justice and fighting injustice, no matter what its incarnation—a message that strongly influenced muslims as well no muslims to uprise against injustice and oppression.
In the first Arba'een gathering in the year 62 AH, Jabir ibn Abd-Allah, a companion of the Prophet, was one of the people who performed a pilgrimage to the burial site of Imam Hussain (as). Due to his infirmity and probable blindness, he was accompanied by Atiyya bin Saad. His visit coincided with that of the surviving female members of the Prophet's family and Husayn's son and heir Imam Zain-ul-Abideen (as), who had all been held captive in Damascus by Yazid (la), the Umayyad Caliph. Imam Zain-ul-Abideen had been too ill to participate in the Battle of Karbala. He later devoted his life to Azadari and spreading the message of Imam Hussain's supreme sacrifice.