Ashura ("the tenth day," Arabic) commemorates the death of the Prophet's 2nd grandson, Hussain, in Karbala at the culimination of the 10 day seige of Hussain and his 72 followers by the Ummayid Caliph's 30,000 strong army. Islam was undergoing a sectarian moment in 61 AH (680 AD), just about 40 years after the Prophet Mohammed's death. Yazid required the family of the Prophet to pledge fealty to him in order to solidify his rule. The martyrdom only solidified the basis for the schism between Sunnis and Shia (followers of the descendants of the Prophet). Modern commemoration during the Islamic month of Moharram lasts ten days with a passion play and the mourning of the death of most of the prophet's family.

Ashura (asor in Hebrew, also "the tenth") corresponds to the Jewish festival of Yom Kippur and is also the traditional date in Islamic theology when Noah's Ark came to rest. Due to the closeness of the Jewish and Islamic communities back in the day, the 10th day of the 7th month is a recommended day of fasting as per the Quran (2:183-187): "[keeping] the fast as it was prescribed for those before you". Here the Prophet is seen as encouraging muslims to adopt fasting on the Day of Atonement. Numerous sayings attributed to the Prophet(called hadith, via Bukhari) mention the event when the Prophet came to Medina and found the Jews fasting and ordered his followers to do the same. Many Sunnis observe this fast.

More Info: The Fast of Ashura (ummah.net), Ashura (google), Viewpoint: Ashura (BBC)

Also, unfortunately, Moharram's become a time when Sunnis and Shia clash.

Hangu, North-West Frontier Province, Pakistan 33.5533, 71.060 (33.5327778, 71.06) 2690 ft., pop. 22974

A suspected suicide bombing on minority Shi'ite Muslims in Pakistan killed at least 23 people, wounded dozens and triggered violence on an important holy day that killed at least four more, police said.
The attack targeted a procession in the town of Hangu in North West Frontier Province to mark Ashura, the holiest day for Shi'ites. Officials reported several blasts. (Reuters)
Violence Mars Ashura Festival, CNN

Herat, Afghanistan 34.345, 62.200 (34.345, 62.1997222) 3064 ft., pop. 4159

Hundreds of Shiite Muslims and Sunnis clashed in a western Afghan city Thursday during an important Shiite festival, hurling grenades and burning mosques, officials and witnesses said. At least four people were killed and 51 injured. (ABC/AP)

Previously, I'd posted a list of violence against Shia, powerlessness.

Incidentally (and somewhat related due to him being not only Jewish but also requiring punches to the facionekal region), today's Jack's birthday.