The Remembrance of Muharram is an opportunity for Muslims to pay respect and mourn the sacrifices made in the Battle of Karbala.
In particular, the mourning period commemorates the death of Husayn ibn Ali, the grandson of the Prophet Muhammad.
Ashura marks the tenth day of Muharram, which is the first month in the Islamic calendar, starting off the new year.
Ashura culminates in this period of remembrance, with Shia and Sunni Muslims observing the event in different ways.
This year, Ashura begins in the evening of Thursday, September 20, and ends in the evening of Friday, September 21.
Because the Islamic calendar is lunar, this is marked as one day, as it is a 24-hour period, but not as marked in the traditional Gregorian calendar.
Do you have to fast?
Fasting is not compulsory during Ashura, but some choose to.
Sunni Muslims regard fasting on Ashura as recommended, but not obligatory.
While fasting during the month of Ramadan (the ninth month in the Islamic calendar) became obligatory, the fast of Ashura was made non-compulsory.
Many do choose to fast and Prophet Muhammad was said to have fasted on the day of Ashura.
If you do choose to fast, it won’t be for two days as Ashura is only a 24-hour period, one day in the Islamic calendar.
How else can you commemorate Ashura?
Sunni and Shia Muslims commemorate this special day in different ways.
There are matam ceremonies of self-flagellation (beating ones own chest with whips or branches).
Others will forego this and instead opt to donate blood.
In some areas, such as in the Shia suburb of Beirut, communities organise blood donation drives with organisations like the Red Cross as a replacement for self-flagellation rituals.
Millions of Muslims around the world will go to worship, with some Mosques offering free meals.
In some places, such as Iran and Iraq, passion plays known as Ta’zieh are performed, reenacting the Battle of Karbala.
Some will make a pilgrimage to a shrine in Karbala, where the tomb of Husayn ibn Ali lies.
The traditions are usually solemn, and there it is prohibited to plan ant wedding or parties on or near the date of Ashura.