The tenth day of the holy month of Muharram will fall on Saturday November 24. The tenth day is also known as Ashura' عاشوراء
Fasting was legislated on Ashura'. Actually, fasting on this day was mandatory upon Muslims even before the fasting of the month of Ramadan. However, later when fasting the whole month of Ramadan was made an obligation, fasting on the day of Ashura' became recommended.
Aisha said, "The tribe of Quraysh used to fast on the day of Ashura' in the Pre-Islamic Period of Jahilliyyah, and Allah's Messenger used to fast on that day too and ordered others to fast as well. Later when the fasting of the month of Ramadan was prescribed, he gave up fasting on the day of Ashura' and it became optional for one to fast on it or to leave its fast." [Recorded by Imam Buhkari and Imam Muslim]
During the life of the Prophet , the Jews and the Christians used to fast on the day of Ashura', out of respect and reverence, because it was the special day on which Allah saved Musa and his followers from Pharaoh and his soldiers. Prophet Musa fasted on that day as a way of showing his gratitude to Allah for saving him and his followers.
Ibn Abbas said, "The Prophet came to Al-Madinah from Mecca and saw the Jews fasting on the day of Ashura' and asked them about their fasting. They replied, "This is a good day, the day on which Allah rescued the children of Israel from their enemy. So, Musa fasted on this day." Therefore, the prophet fasted on that day and ordered (the Muslims) to fast on that day as well." [Recorded by Imam Buhkari and Imam Muslim]
In another narration the Prophet said: "If I live to next year I will fast the ninth day (as well as the tenth)." [Recorded by Imam Muslim and Imam Ahmad]