FACTBOX-What is behind Ashura violence in Pakistan?

(Reuters) - At least 11 people died in a suicide bomb blast in a Shi'ite Muslim prayer hall in the Pakistani city of Peshawar on Thursday.

The blast comes one day before the Ashura weekend, the Shi'ite calendar's biggest event, which has become a flashpoint for deadly attacks by Sunni militants over recent years.

Here are some key facts about Ashura and frayed relations between Pakistan's Sunni and Shi'ite groups:

WHAT IS ASHURA?:

-- Ashura falls on the tenth day of a 40-day mourning period during the Islamic calendar's first month, Moharram, which commemorates the death of Imam Hussein, a grandson of the Prophet Mohammad, who was killed in battle in A.D. 680 in the Iraqi city of Kerbala.

-- The schism between Sunnis and Shi'ites is rooted in a dispute over the successors of the Prophet following his death in A.D. 632.

-- Sunnis regard Abu Bakr, one of Mohammad's companions, as his successor. Shi'ites revere Ali, the prophet's son-in-law and cousin. Ali was also the father of Hussein, whose death Ashura commemorates.

-- Marching down streets, worshippers flog themselves with steel-tipped flails or slash their bodies with knives to express solidarity with Hussein.

WHY IS THE FESTIVAL A FLASHPOINT FOR VIOLENCE?: