Arba'een (Arabic: الأربعين‎, romanized: al-Arba‘īn, lit. 'forty'), Chehlom (Persian: چهلم‎, Urdu: چہلم‎, "the fortieth day") is a Shia Muslim religious observance that occurs forty days after the Day of Ashura. It commemorates the martyrdom of Husayn ibn Ali, the grandson of the Islamic Prophet Muhammad, who was killed on the 10th day of the month of Muharram. Imam Husayn ibn Ali and 72 of his companions were killed by Yazid's army in the Battle of Karbala in 61 AH (680 CE).

Arba'een or forty days is also the usual length of mourning after the death of a family member or loved one in many Muslim traditions. Arba'een is one of the largest pilgrimage gatherings on Earth, in which up to 45 million people go to the city of Karbala in Iraq.^{[3][4][5][6][7]}

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 in Shia Islam.

According to tradition, the Arba'een 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 Arba'een pilgrim, made a pilgrimage to the burial site of Husayn.^[8] He was accompanied by Atiyya ibn Sa'd because 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.^[9] 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,^[1] 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?’^[2][10][11]

Arba'een'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 Arba'een publicly in Iraq. Following the 2003 invasion of Iraq, the observance in April 2003 was broadcast worldwide.^[12]

The city of Karbala in Iraq is the center of the proceedings which many pilgrims travel miles on foot to reach. As of 2016 “between 17 million and 20 million” pilgrims usually attend Arba'een there, including about three million foreigners, most of whom are Iranians.^[13]

Arba'een is consistently among the largest peaceful gatherings in history. Every year, huge crowds of pilgrims travel to the city of Karbala in pilgrimage to the Imam Hossein holy shrine in Karbala on Arba’een Day.^[14] (For example it is over 500 km from Basra the largest city in southern Iraq where Shia predominate to Karbala.)^[15] It is traveled annually on foot by Iraqi pilgrims, which takes them two weeks, or approximately one month to come from other countries like Iran. The crowds become so massive that roads are blocked for hundreds of miles.

In 2008, approximately nine million religious observers converged on Karbala to commemorate Arba’een.^[16] In 2009, over ten million people were estimated to have reached Karbala, according to BBC News and Press TV. In 2013, 20 million pilgrims from 40 countries came for Arbaeen, according to Iranian media.^[17][18][19] A car bomb targeting worshippers returning from Karbala killed at least 20 Shiite pilgrims in January 2013.^[20] In 2014, up to 17 million people made the pilgrimage and many choose to make the 55-mile journey on foot from Najaf, near areas controlled by the militant Islamic State of Iraq and the Levant (ISIL), which has declared Shia Muslims apostates.^[21][22][23] Up to 17 million pilgrams came in 2015^[24] and 2016

The Ziyarat Arba'een is a prayer which is usually recited in Karbala on the day of Arba'een. It is narrated from Safwan al-Jammaal from Imam Ja'far al-Sadiq, the sixth Shiite Imam, in which the Imam instructed him to visit Imam Husayn's mosque, and to recite a specific visitation prayer on Arba'een by which the believer should reaffirm their pledge to Husayn's ideals. The Ziarat or prayer is a text which designates Husayn as the "inheritor" of the prophets Adam, Noah, Abraham, Moses, Jesus and Muhammad.

While the Arba'een is a distinctively Shi'a spiritual exercise, Sunni Muslims and even Christians, Yazidis, Zoroastrians, and Sabians partake in both the pilgrimage as well as serving of devotees. Pilgrims from European countries including Sweden, Russia and even a delegation from Vatican City have joined in past observances. Some Iraqi Christian religious leaders also joined the delegation from the Vatican.^[26][27]

Many delegations from various African countries including Ghana, Nigeria, Tanzania and Senegal have also participated in the Arba'een.^[19]

Since the first Arba'een, it has influenced subsequent Shi'ite uprisings against Umayyad and Abbasid rule. Arba'een has also been used as a political protest, at least in Iran. It was first used there to protest the killing of supporters of Ayatollah Ruhollah Khomeini in Qom on 5 June 1963 when a general strike was announced. A cycle of Arba'een public observance of mourning rituals of martyred protestors — where an Arba'een observance was held to commemorate those killed in the preceding Arba'een protest demonstration — is often credited as part of the reason for the success of the 1979 Iranian Revolution that overthrew Shah Mohammad Reza Pahlavi,^[29] although that explanation has also been questioned.^[30]

While Arba'een is always on nearly the same day (20 or 21 Safar) of the Islamic calendar, the date on the Gregorian calendar varies from year to year because of differences between the two calendars, since the Islamic calendar, the Hijri calendar (AH), is a lunar calendar and the Gregorian calendar is a solar calendar. Furthermore, the method used to determine when each Islamic month begins varies from country to country (see Islamic calendar).

Arba'een always falls 40 days after the Day of Ashura. The Day of Ashura, in turn, falls nine days after the first day of Muḥarram. Hence, Arba'een falls 49 days after the first day of Muḥarram. This date is shown for a selection of years, according to the Umm al-Qura Calendar of Saudi Arabia, in the table below: