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Pakistani Shi'ite Muslims flagellate themselves during an Ashura procession in PeshawarAFP

On Friday (23 October), millions of Muslims will gather to mark the holy day of Ashura, the tenth day of Muharram in the Islamic calendar. The 10-day festival of Muharram is observed by both Sunni and Shia sects, but for Shia Muslims it is also a major commemoration of the assassination of Imam Husayn, a Shia leader and the grandson of the Prophet Mohammed.

Husayn was killed by the army of the Caliph Yazid at the Battle of Karbala in 680 AD, in what is now modern-day Iraq. His death was an event that led to the split in Islam into two main sects - Sunnis and Shias. During Ashura, some Shia men flagellate themselves with chains, beat their heads and chests with chains and knives, and gash their heads with swords to show their grief and echo the suffering of Husayn in a ritual called "tatbir".

This controversial practice is discouraged by some Shia leaders, who say it creates a negative image - but is still practised by many. On Ashura, some Shia men and women parade through the streets dressed in black, slapping their chests and chanting, while others reenact the Battle of Karbala with mourning rituals and passion plays. Many also choose the holy day to make a pilgrimage to the Mashhad al-Husayn, the shrine in Karbala in Iraq that is regarded as Husayn's tomb.

Ashura will be marked during an especially turbulent time in the Middle East, which has seen an increase in sectarian violence alongside the spread of Islamic State (Isis) and other extremist Sunni groups, who believe Shias are apostates deserving of death. A suicide bombing on Thursday (22 October) at a Shia mosque in the Pakistani province of Baluchistan killed at least 10 people, sparking fears of more attacks during the upcoming holiday.

On 16 October, two people were killed in a shooting attack at a Shia mourning ceremony in Iran after assailants opened fire at the worshipers from a car. In a similar attack that took place at a Shia gathering in Saudi Arabia the same day, five people were killed and nine were injured by a gunman. A group claiming links to Islamic State group reportedly took responsibility for the attack.