Last Prayer of Imam Hussein - Day of Ashura
The Day of Ashura is on the 10th day of Muharram in the Islamic calendar and marks the climax of the Mourning of Muharram.
It is commemorated by Shi'a Muslims as a day of mourning for the martyrdom of Husayn ibn Ali, the grandson of Muhammad at the Battle of Karbala on 10 Muharram in the year 61 AH. According to Sunni Muslim tradition, Ibn Abbas narrates that The Prophet ﷺ came to Madeenah and saw the Jews fasting on the day of ‘Ashoora’. He said, “What is this?” They said, “This is a good day, this is the day when Allah saved the Children of Israel from their enemy and Moosa fasted on this day.” He said, “We are closer to Moosa than you.” So he fasted on this day and told the people to fast.
This day is well-known because of mourning for the martyrdom of Husayn ibn Ali, the grandson of Muhammad and the third Shia Imam, along with members of his family and close friends at the Battle of Karbala in the year 61 AH (680 AD). Yazid I was in power then and wanted the Bay'ah (allegiance) of Husayn ibn Ali. Muslims believe Yazid was openly going against the teachings of Islam in public and changing the sunnah of Muhammad. (Source)