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Arba'een (Arabic: الأربعين, "forty"), Chehelom (Persian: چهلم, Urdu: چہلم, "the fortieth [day]") or Qirkhi, Imamin Qirkhi (Azerbaijani: İmamın qırxı, امامین قیرخی, "the fortieth of Imam") 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 Muhammad, which falls on the 20th day of the month of Safar. Imam Husayn ibn Ali and 72 companions were killed by Yazid I'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 over 31 million people go to the city of Karbala in Iraq.
The Arba'een pilgrimage has been observed since the year 61 AH of the Islamic calendar (October 10, 680), after the Battle of Karbala, or the following year. According to tradition, the first such a gathering took place when Jabir ibn Abd-Allah, a companion of Muhammad, made a pilgrimage to the burial site of Husayn. He was accompanied by Atiyya bin Saad due to his infirmity and probable blindness. His visit coincided with that of the surviving female members of Muhammad's family and Husayn's son and heir Imam Zain-ul-Abideen, who had all been held captive in Damascus by Yazid, the Umayyad Caliph. Imam Zain-ul-Abideen had survived the Battle of Karbala and led a secluded life in deep sorrow.It is said that 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,[a] 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?’[b] 
Arba'een's performance has been banned in some periods, the last of which was when Saddam Hussein, 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.
Arba'een is consistently among the largest peaceful gatherings in history. The city of Karbala in Iraq is the center of the proceedings to which many pilgrims travel miles on foot to reach there. The distance between Basra and Karbala is a long journey[clarification needed] even by car, but 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 they cause a blockade for hundreds of miles. In 2008, approximately nine million religious observers converged on Karbala to commemorate Arba’een. However, in 2009, the number of people visiting Karbala on Arba'een significantly increased. According to BBC News and Press TV, over ten million people had reached Karbala one or two days before Arba'een. The number of pilgrims was expected to rise to 18 million during the next two days; Arbaeen reached 20 million in 2013.
Kumbh Mela is more populous than Arba'een, but it is not held annually.
The Ziyarat Arba'een is a prayer which is usually recited in Karbala on the day of Arba'een. It is been 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 believer should reaffirm their pledge to Husyin's ideals. The Ziarat or prayer is a text which designates Husayn as the "inheritor" of Adam, Noah, Abraham, Moses and Jesus.
Other religions and countries in the Arba'een
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.
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 June 5, 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, although that explanation has also been questioned.
Arba'een in the Gregorian calendar
While Arba'een is always on the same day of the Islamic calendar, the date on the Gregorian calendar varies from year to year due to 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).
- 2005 (1426 AH): March 31
- 2006 (1427 AH): March 21
- 2007 (1428 AH): March 10
- 2008 (1429 AH): February 28
- 2009 (1430 AH): February 15
- 2010 (1431 AH): February 5
- 2011 (1432 AH): January 25
- 2012 (1433 AH): January 14
- 2013 (1434 AH): January 3
- 2013 (1435 AH): December 23 (Dec 24, if the month of Muharram is 30 days); according to Makkah 
- 2014 (1436 AH): December 13
- Quran, 12:84
- From Shaykh as-Sadooq, al-Khisal; quoted in al-Ameen, A’yan, IV, 195. The same is quoted from Bin Shahraashoob’s Manaqib in Bih’ar al-Anwar, XLVI, 108; Cf. similar accounts, Ibid, pp. 108-10
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- Vali Nasr, The Shia Revival. New York: Norton, 2006; pp 18–19.
- Official website of Multi-National Force - Iraq
- "زيارة الاربعين: 18 مليون زائر ونجاح امني كبير". Al-Alam. Retrieved 2013-01-04.
- http://z313.ir/132//%D9%85%D9%82%D8%A7%D9%84%D8%A7%D8%AA/%D8%A7%D8%B1%D8%A8%D8%B9%D9%8A%D9%86-%D8%8C%D9%85%D9%8A%D8%B9%D8%A7%D8%AF%DA%AF%D8%A7%D9%87-%D8%B3%D9%BE%D8%A7%D9%87-%D8%A7%D9%85%D8%A7%D9%85-%D8%B9%D8%B5%D8%B1(%D8%B9%D8%AC)-%D8%AF%D8%B1-%D9%87%D9%86%DA%AF%D8%A7%D9%85-%D8%B8%D9%87%D9%88%D8%B1/ , December 2014
- http://www.independent.co.uk/news/world/middle-east/one-of-the-worlds-biggest-and-most-dangerous-pilgrimages-is-underway-9882702.html December 2014
- Al-Modarresi, Mahdi. "World's Biggest Pilgrimage Now Underway, And Why You've Never Heard of it! huffingtonpost". Retrieved 2014-12-11.
- "Christians in Karbala in Arbaeen". Retrieved 2014-12-11.
- "Millions of Shia Muslims from across the globe have come together in the Iraqi city of Karbala to mark the Arbaeen ritual, which marks the 40th day following the seventh-century martyrdom of the third Shia Imam, Imam Hussein, Press TV reports.".
- Kurzman, Charles, The Unthinkable Revolution in Iran, Harvard University Press, 2004, p.54-5
- Kurzman, The Unthinkable Revolution in Iran, (2004), p.57