The Day of Ashura (Arabic: عاشوراء‎ ʻĀshūrā’, Ashura, Ashoura, and other spellings; Turkish: Aşure Günü) is on the 10th day of Muharram in the Islamic calendar and marks the climax of the Remembrance of Muharram.
It is commemorated by Shia Muslims as a day of mourning for the martyrdom of Husayn ibn Ali, the grandson of the Islamic Prophet Muhammad at the Battle of Karbala on 10 Muharram in the year 61 AH (October 2, 680 CE). According to Sunni Muslim tradition, Muhammad fasted on this day and asked other people to fast. Sunni Muslims also remember the day claiming that Moses fasted on that day to express gratitude to God for liberating the Israelites from Egypt.
In some shia regions of Muslim countries such as Afghanistan, Iran, Iraq, Azerbaijan, Pakistan, Lebanon, and Bahrain, the Commemoration of Husayn ibn Ali has become a national holiday and most ethnic and religious communities participate in it. Even in predominantly Hindu country like India, Ashura (often called Moharram) is a public holiday.

en.wikipedia.org/wiki/Day_of_Ashura