When do we fast Ashura?

Fasting the day of Ashura is recommended and a Sunnah, and it expiates the sins of the past year. It is the tenth day of the month of Muharram, by agreement of the scholars. Ibn Abbas's statement about fasting the ninth day should not be understood to mean that Ashura is the ninth day; rather, he guided the questioner to fast the ninth along with the tenth, in opposition to the Jews. There are three levels of fasting Ashura: the most complete is fasting the day before it and the day after it, then fasting the ninth and the tenth, then fasting the tenth alone.