Tomorrow (which starts at sundown tonight) is the tenth of the month of Muharram (the first month of the year) on the Islamic calendar, also known as Ashura (ashr = 10). This day is commemorated by some Sunnis and all Shiʿa with fasting and ritual. For Sunnis, Ashura is traditionally (going back to Muhammad) observed as the day when the Israelites were freed from Egypt, though it also has overtones of Yom Kippur. Muslims recognize this as a “day of atonement” (yawm al-ghufran), which should remind you of Yom Kippur, both in the Arabic words used and in the fact that Yom Kippur similarly comes around once a year on the tenth day of the first month of the Jewish calendar. Muhammad is said to have fasted for two days to commemorate Ashura, and recommended (but did not require) that his followers do the same. Many, though not all, Sunnis observe this fast today.
For Shiʿa, the day has a much deeper meaning, as it was on Ashura in the year 61 (~October 10, 680 for those of us who use the Gregorian calendar) when Imam Husayn, Ali’s son, was killed in battle with the Umayyad army at Karbala, in modern Iraq. Many (though, again, not all) Shiʿa participate in some way in a mix of commemorative activities that have developed around Ashura over the centuries. They may undertake a pilgrimage to Husayn’s burial shrine in Karbala, participate in a reenactment of the battle, fast, and/or join in public mourning, which can include chanting and poetry readings but also can include self-mortification, either chest-beating or striking themselves with chains and even blades. This self-violence is often discouraged or even prohibited by religious authorities, and its practice annually generates discussion within the Shiʿa community about whether this is actually appropriate and what kind of an image it presents of Shiʿism, etc., but these rituals are centuries old and they’re not going anywhere.
Buzzfeed collected a number of amazing Ashura photographs in 2013; all are worth checking out (though be warned, some of them are pretty graphic).
I should add that many Sunnis, who honor Muhammad’s family even though they don’t accept the Shiʿa argument that Muhammad’s descendants should rule the Muslim community, also recognize this day for Husayn’s martyrdom, and some may even participate in these rituals (historically that has been the case, at least in Iran). Unfortunately, these commemorations can also become scenes of anti-Shiʿa violence, or the commemoration of the day at all can be used as a tool by Sunni political authorities to repress Shiʿa communities under their control. We’ve already seen examples of both this year, including an attack on an Ashura procession in Pakistan’s Sindh province that has killed at least 16 people and injured another 30, a day after a bombing at a Shia mosque in Baluchistan province killed 10 people.