The holiest day for Shi'a Muslims is celebrated on the 10th day, or 'Ashurah, of the Muslim month of Muharram. This year that equates to Saturday, February 19.
The story -
Shi’a Muslims make up almost 20 percent of the of the world’s second largest religion. The difference between the sects is as old as the religion itself and revolved around the issue of succession – who would follow Muhammad as the leader of the faithful? The Arabic word for “one who follows or succeeds,” - khalifah (Caliph) - was adopted as the title.
Many believed that the successor to Muhammad should be a family member, someone in the bloodline of the Prophet. However, Muhammad had no son, so there was no male heir to the caliphate. Muhammad did have a daughter, Fatima, who was married to Muhammad’s cousin ‘Ali bin Abu Talib. The people who favored the selection of ‘Ali as the caliph were called the Shi’at ‘Ali, the “partisans of ‘Ali,” and hence the name Shi’a.
The other school of thought, held by many prominent Muslims of the day, was that the caliph should be drawn from one of the senior and learned members of the faith, the ummah or “community.” These were the Sunnis, the traditionalists.
The Sunni position prevailed and the first three caliphs were not of Muhammad’s bloodline. Finally, a convergence occurred in 656 when ‘Ali (regarded by the Shi’a as the first Imam) was named the fourth Caliph. ‘Ali was soon murdered and his son Hasan became the second Imam. However, real political power at this time rested with the Sunni caliph in Damascus. Hasan abdicated in favor of these ‘Umayyad rulers.
Hasan’s brother Husayn assumed the Shi’a imamate, presaging what became the major divide in Islamic history. In 680, Husayn was killed in battle against superior ‘Umayyad forces in Karbala’, Iraq on the tenth day of the month of Muharram. This day is commemorated by all Shi’a as ‘Ashurah (literally, “the tenth”) as a day of mourning and perfidy on the part of the Sunnis. Many male Shi’a practice self-flagellation to honor the death of the imam.