It is observed each year across the Muslim world on the 10th day of Muharram, the first month of the Islamic year, and is a public holiday in Iran, Iraq, Afghanistan, Pakistan and other countries with large Shiite populations.
The events and rituals surrounding Ashura are colorful and dramatic, with huge banners hung from poles, black-draped archways erected across roadways, vivid posters depicting the life and death of Hussein, and martial dirges blaring from loudspeakers in hypnotic, pounding rhythms.
Emotional and religious fervor build throughout the day, leading to the bloody and controversial ritual in which groups of Shiite men and boys flagellate themselves with small knives and chains, and many others beat their chests for hours.
The defeat and death of Hussein, at the hands of forces loyal to Yazid, a caliph from the Arab Omayyad dynasty, was the seminal event that led to the division of Islam into Sunni and Shiite sects, with the major difference relating to the line of succession after the death of the prophet and lesser differences over issues of ritual and prayer. That split has come to divide the Middle East into camps dominated by Shiite Iran and Sunni Saudi Arabia, and it has led to sectarian violence in countries such as Iraq and Pakistan. Sunnis also observe Ashura, but as a day of gratitude and fasting in honor of the prophet Moses, rather than a day of sorrow and public mourning for Hussein.
Here's how nations around the world mark Ashura:
(Editor’s note: Some images in this post may be disturbing because of their violent or graphic nature.)