Every year, on the twentieth of Safar, also known as Arba'in, millions of pilgrims flock to Karbala, Iraq, often arriving there on foot from the nearby city of Najaf.

On this day in 61 AH (680 CE), Husayn was killed, alongside most of his relatives and his small retinue, in the Battle of Karbala against the army of the Umayyad caliph Yazid ibn Mu'awiya (r. 680–683).

In Shia Islam, Karbala symbolizes the eternal struggle between good and evil, the pinnacle of self-sacrifice, and the ultimate sabotage of Muhammad's prophetic mission.

[5] Husayn was killed on 10 Muharram 61 AH (10 October 680 CE), alongside most of his male relatives and his small retinue, in the Battle of Karbala against the army of the Umayyad caliph Yazid ibn Mu'awiya (r. 680–683), having been surrounded for some days and deprived of the drinking water of the nearby Euphrates river.

[6][7][8] The fight took place in the desert land of Karbala, en route to the nearby Kufa, whose residents had earlier invited Husayn to lead them against Yazid.

[9] In Shia Islam, Karbala symbolizes the eternal struggle between good and evil,[10][11] the pinnacle of self-sacrifice,[12] and the ultimate sabotage of Muhammad's prophetic mission.

[13] Historically, the event served to crystallize the Shia community into a distinct sect and remains an integral part of their religious identity to date.

[23] Probably by combining the accounts available to him,[23] the Shia scholar Ibn Tawus (d. 1266) reports that Husayn's relatives returned via Karbala to their hometown of Medina when they were freed from captivity in Damascus.

Upon arrival in Karbala on Arba'in,[25][26] they met Jabir ibn Abd Allah (d. 697), a companion of Muhammad,[23] who had learned about the death of Husayn through a divine sign.

[23] The veracity of Ibn Tawus' account has therefore been questioned by some, including the Shia scholar Husain Noori Tabarsi (d. 1902) and the Islamicist Mahmoud M. Ayoub (d. 2021).

[23][4] Ayoub adds that Arba'in is not mentioned in Kamil al-ziyarat, an early and authoritative hadith collection by the Shia traditionist Ibn Qulawayh (d. c. 978).

[39] Even though the Hindu festival Kumbh Mela draws a larger crowd, it is held once every three years, which makes the Arba'in pilgrimage "the world's largest annual gathering in one place.

It increasingly attracts participants from various religious backgrounds, including Sunnis, Christians, Jews,[43] and non-Abrahamic faiths such as Hindus, Yazidis, and Zoroastrians, who come to commemorate and mourn the death of Husayn.

[59] In the same year, the Iraqi police seized eighteen booby-trapped dolls, stuffed with explosives, which were intended to be scattered on the roads leading to Karbala during the Arba'in pilgrimage.

[64][65][66][67] In 2016, Asharq al-Awsat, a Saudi-owned news site, reported from the World Health Organization (WHO) that "unplanned pregnancies and [...] disease" were seen "following the arrival of scores of unregulated Iranians to take part in the annual Shia pilgrimage to Karbala."