Typically Muslims fast during the month of Muharram and those people who cannot fast the entire month will do so on the 10th day, considered to be the most sacred day of the month. These fasts are not required and the number of days of fasting depends on the person. Those who choose to fast are believed to be entitled to a reward from Allah.
Muharram is the first of the 12 months of the Islamic calendar. It is the most sacred month after Ramadan and is a time for remembrance. Since the Islamic calendar is a lunar calendar, the exact dates move each year.
The 10th day of Muharram is called Ashurah. Muslims believe that this is the day when Moses crossed the Red Sea, receiving victory over the Egyptian pharaoh.