One of the most important dates for Shia Muslims is the 10th of the Muslim month of Muharram, which falls on September 20 this year. It marks the culmination of the 10-day mourning period for the killing of Imam Hussein, the grandson of the Prophet Muhammad in the bloody Battle of Karbala in 680 AD.
Hussain, his relatives' and followers' tragic end in the fight against the Ummayad leader Yazid and his armies over the Muslim Caliphate laid the foundation for the faith practised by the Muslim Shia community.
What does Muharram mean for Muslims?
Muharram is the first month of the traditional Islamic calendar.
It is also seen as the "month of remembrance" of those lost in the Battle of Karbala. There are solemn and commemorative events rather than joyful celebrations held in most Muslim countries throughout the period, in some cases extending to 40 days.
But the first 10 days are considered the most important.
What happens on Ashura?
Shia Muslims across the world mark the days leading up to Ashura each year starting from Muharram 1.
While Sunni Muslims also acknowledge the events of Karbala, many do not observe the month of remembrance or Muharram 9th or 10th with the same intensity.
On the days of Ashura, Shias across the world express their mourning in various ways.
They gather at mosques and shrines, or in processions for ceremonial mourning that involves the ritualistic "matam" or beating of their chests, slapping their faces and self-flagellation to reflect the grief over the violent deaths of Hussein and his family.
People who participate in the processions or convoys in the streets carry colourful flags and sing hymns dedicated to Hussein.
Millions of Shias from Iraq, Iran, Pakistan, Azerbaijan, Bahrain, Lebanon and beyond also flock every year to Karbala in Iraq to mourn Hussein's death and observe Arabaeen, the 40th day of mourning.
Where is celebrating Ashura an issue?
Shia Muslims in India-administered Kashmir have found it near impossible to observe Muharram – up until the mid-1900s and later since 1989 when the rise of the Muslim separatist movement prompted the Indian government to ban Ashura processions.
Authorities often place barriers and impose curfews in the major cities in the days ahead of Ashura.
On Wednesday, police fired tear gas and used batons to break up religious processions in the main city of Srinagar.
Police detained over a hundred mourners, who were expected to be released later in the day.
Indian authorities "provide state facilities and protection to facilitate the annual Hindu pilgrimage in Kashmir but ban our traditional religious processions," said Hakim Adil, a participant in Wednesday's processions.
Over the years, Shia Muslims and Ashura processions have been targeted by terror groups in Afghanistan.
This year after terror groups targeted Shia Muslims multiple times, killing dozens, Afghan police and volunteer civilian groups tightened controls across the capital Kabul ahead of Ashura.
Armed men stood at key points in areas like Dasht-e Barchi, a zone in western Kabul where many members of the mainly Shia Hazara minority live and where some 20 people were killed in a suicide attack on a wrestling club two weeks ago.