But the emphasis differs according to the branch of Islam to which one adheres. It has been a day of fasting since the early days of Islam. It recognizes the day that Nuh -- Noah in Judeo-Christian scripture -- left the Ark, and the day that Allah saved Musa -- Moses -- from the Egyptians. Pious Sunni still observe it as such with fasting and prayers.
For Shi'a Muslims, however, it is a day to remember the suffering and martyrdom in the Western calendar's year 680 of Imam Hussein, grandson of the prophet Muhammad.
The Shi'a hold that Hussein's father was the true successor to Muhammad as the leader of Islam, and that when Hussein sought to assume the leadership he was pursued to Karbala in what is now Iraq and suffered a brutal death in battle there. This event cemented the split of Islam.
Faithful Shi'a commemorate Hussein's martyrdom on Ashura and the week leading up to it with passionate acts of contrition and mourning. Men weep and march in processions beating themselves bloody with chains and knives.
Former Iraqi leader Saddam Hussein and his Sunni followers suppressed observation of Ashura during their reign. With Hussein gone, Shi'a -- who are the majority in Iraq -- have revived the ceremonies there.
Similarly, the Taliban rulers of Afghanistan suppressed Ashura observances. Afghanistan's minority Shi'a resumed recognition of the holy day in 2002 after U.S.-led forces overcame the Taliban.
In most places where Sunni and Shi'a live in proximity they live in peace. But radical groups from both branches of Islam often choose Ashura as a time for deadly attacks.
Militants killed 170 people last year in simultaneous attacks on Ashura pilgrims in Baghdad and Karbala last year. Karbala police are braced this year to prevent such a recurrence. Jawad Nasir al-Shimmari is a Karbala police official.
"We do not fear death or bombings and by God's will and the wise leadership of Major General Abbas Fadhil Abood [Iraq's chief of police] and head of the city's police department we will have full control of the situation."
Authorities blamed last year's attacks on the lack of police to secure the area, on passivity of coalition forces in the region, and on infiltration by terrorists hiding in crowds of foreign pilgrims.
This year Iraq closed its borders for five days preceding Ashura. Iraqis have set up a three-tiered defense -- international forces, the Iraq army, and local police. The French news agency AFP reports that police guarding access to shrines in Karbala are even turning away children's bicycles.
Even so, two bomb blasts near Shi'ite mosques in Baghdad today killed at least 15 people and wounded at least 24 others. The first blast appears to have been a suicide bombing.
Police in neighboring Pakistan -- to prevent Ashura attacks -- have detained 50 men they suspect as members of a Sunni extremist group.
In the Pakistani city of Quetta police battled with two militants today near the planned route for an Ashura procession on 20 February. The shootout ended when the two blew themselves up with a powerful bomb. Nobody else was injured.
(Radio Free Europe's Radio Farda provided audio for this feature.)