Arbaʽeen or forty days is also the usual length of mourning after the death of a family member or loved one in many Muslim traditions. Arbaʽeen is one of the largest pilgrimage gatherings on Earth, in which up to 45 million people go to the city of Karbala in Iraq.

The significance of the number 40 has roots in a saying (hadith) of Muhammad: “On the day of judgment, among my people, God will consider whoever memorized forty Hadiths as an erudite man”.

Numerous Islamic scholars have gathered collections of forty hadith, quoting from the prophet and the Imams, who followed him through the Shia sect.

According to tradition, the Arbaʽeen pilgrimage has been observed since the year 61 AH of the Islamic calendar (10 October 680) after the Battle of Karbala or the following year. According to tradition, the first such gathering took place when Jabir ibn Abd Allah, a sahabah and the first Arbaʽeen pilgrim, made a pilgrimage to the burial site of Husayn. He was accompanied by Atiyya ibn Sa’dbecause of his infirmity and probable blindness. According to tradition, his visit coincided with that of the surviving female members of Muhammad’s family and Husayn’s son and heir, Imam Ali ibn Husayn Zayn al-Abidin (also spelled Zain-ul-Abideen), who had all been held captive in Damascus by Yazid I, the Umayyad Caliph.

Zayn al-Abidin had survived the Battle of Karbala and led a secluded life in deep sorrow. He lived under pressure and tight surveillance set by Umayyad Caliphate. According to legend, for twenty years whenever water was placed before him, he would weep. One day a servant said to him, ‘O son of Allah’s Messenger! Is it not time for your sorrow to come to an end?’ He replied, ‘Woe upon you! Jacob the prophet had twelve sons, and Allah made one of them disappear. His eyes turned white from constant weeping, his head turned grey out of sorrow, and his back became bent in gloom, though his son was alive in this world. But I watched while my father, my brother, my uncle, and seventeen members of my family were slaughtered all around me. How should my sorrow come to an end?’

Arbaʽeen’s performance has been banned in some periods, the last of which was when Saddam Hussein, (a Sunni who ruled as an Arab nationalist, clashing with Islamic revivalism) was president of Iraq. For nearly 30 years under Saddam’s regime, it was forbidden to mark Arbaʽeen publicly in Iraq. Following the 2003 invasion of Iraq, the observance in April 2003 was broadcast worldwide.

Here are some videos on the history.