9th and 10th Ashura
The Prophet (peace and blessings of Allah be upon him) said: “For fasting the day of ‘Ashura’, I hope that Allah will accept it as expiation for the year that went before.” (Muslim). This is from the bounty of Allah towards us: for fasting one day He gives us expiation for the sins of a whole year. And Allah is the Owner of Great Bounty.
Ibn ‘Abbas (may Allah be pleased with him) said: “The Prophet (peace and blessings of Allah be upon him) came to Madinah and saw the Jews fasting on the day of ‘Ashura’. He said, ‘What is this?’ They said, ‘This is a righteous day, it is the day when Allah saved the Children of Israel from their enemies, so Musa fasted on this day.’ He said, ‘We have more right to Musa than you,’ so he fasted on that day and commanded [the Muslims] to fast on that day.” (Bukhari).
“This is a great day, on which Allah saved Musa and his people, and drowned Pharaoh and his people.” (Muslim)