Arba'een (Arabic: اربعين‎, means "forty" , 40), or Chehlum, as it is known by Urdu and Persian-speaking Muslims in Central Asia and south-east Asia, is a Shi'a Muslim religious observation that occurs 40 days after the Day of Ashura, the commemoration of the martyrdom of Husayn bin Ali, the grandson of the Prophet Muhammad which falls on the 20th day of the month of Safar. Husayn and 72 supporters died in the Battle of Karbala in the year 61 AH (680 CE), killed by Yazid's army. Forty days is the usual length of the time of mourning in many 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 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 probable 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 Imam Zain-ul-Abideen, who had all been held captive in Damascus by Yazid I, the Umayyad Caliph. Imam Zain-ul-Abideen (PBUH) had been too ill to participate in the Battle of Karbala.He later devoted his life to Azadari and spreading the message of Imam Hussain's supreme sacrifice.

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 ibn Ali. Many pilgrims travel miles on foot to reach Karbala.

Observance of Arba'een in Karbala was banned for many years when Saddam Hussein, a Sunni Muslim, was president of Iraq. For nearly 30 years under Saddam's regime it was forbidden to mark Arbaeen publicly in Iraq. Following the 2003 invasion of Iraq, the observance in April 2003 was broadcast worldwide.

In 2008, approximately nine million religious observers converged on Karbala to celebrate Arba’een. However, in 2009, the number of people visiting Karbala on Arba'een significantly increased. According to official website of BBC News and Press Tv (Iran), over ten million people had reached the city of Karbala one or two days before Arba'een. The number of pilgrims was expected to rise to 14 million during next two days.

en.wikipedia.org/wiki/Arba'een

www.youtube.com/watch?v=7enbu4vIte4

Why forty days?
Because the number 40 holds tremendous spiritual significance in the realm of transformation. Jesus wandered in the desert for 40 days in order to experience purification and come to a greater understanding of himself and his mission. Moses and his people traveled through the desert for 40 years before arriving at their home in the holy land. Noah preserved the sacredness of life by sailing his ark for 40 days and 40 nights. According to the Kabbalah, the ancient Jewish mystical text, it takes 40 days to ingrain any new way of being into our system...