Arba'een (اربعين, Arabic "forty") is a Shi'a religious holiday that occurs forty days after Aashura, the commemoration of the martyrdom by beheading of Husayn bin Ali, the grandson of the Prophet Muhammad. Husayn and 72 supporters died in the Battle of Karbala in the year 61 AH (680 CE). Forty days is the usual length of the time of mourning in Islamic cultures.

The occasion reminds the faithful of the core message behind Husayn's martyrdom: establishing justice and fighting injustice, no matter what its incarnation -- a message that strongly influenced subsequent Shi'a uprisings against the perceived tyranny of Umayyad and Abbasid rule.

In the first Arba'een gathering in the year 62 AH, Jabir ibn Abdullah, a companion of the Prophet, was one of the people who performed a pilgrimage to the burial site of Husayn. Due to his infirmity and probably 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 Ali, who had all been held captive in Damascus by Yazid I, the Umayyad Caliph. Ali ibn Husayn had been too ill to participate in the Battle of Karbala, but looked on from the sidelines.

The city of Karbala in Iraq, the third holy place of Shi'a Islam, is the center of the proceedings where in a show of humility, many crawl through the streets of the city while others fall on their hands and knees as they approach the Shrines of Husayn and his brother Abbas.

As with all Muslim religious holidays, Arba'een follows the lunar Islamic calendar, not the Western solar Gregorian calendar. Arba'een fell on 31 March in the year 2005.