Ashura procession in Tehran
Ashura falls on the 10th day of Muharram, the first month in the Islamic calendar.
The day is of particular significance to Shia Muslims, who commemorate the martyrdom of Husayn Ibn Ali, an Imam and the grandson of the Prophet Muhammad.
It is considered a day of immense grief and is a national holiday in many countries with large numbers of Shias, such as Iran, Iraq, Lebanon, Pakistan and India.
According to Islamic sources, the tradition of mourning the day of Ashura was started by the sister of Husayn, Zayneb binte Ali.
To commemorate the day, Shia Muslims congregate and engage in sorrowful and poetic recitations that eulogise Husayn and his companions.
According to the Sunni tradition, Muhammad fasted on this day and recommended others to fast too. Sunnis also claim that Moses fasted on this day to give thanks to God for liberating the Israelites from Egypt.