The Day of Ashura is the tenth of Muharram, and it is the day when Allah saved the Children of Israel from Pharaoh, so Moses, peace be upon him, fasted it. It is recommended to fast it along with the ninth of Muharram or the eleventh, as mentioned in the margin of Al-Tahdhib 1: 332; Abu Qatada, may Allah be pleased with him, reported that the Prophet, peace be upon him, said: "Fasting the Day of Ashura, I hope that Allah will forgive the year that came before it," in Sahih Muslim 2: 818. Aisha, may Allah be pleased with her, said: "The Messenger of Allah, peace be upon him, commanded fasting on the Day of Ashura, but when Ramadan was made obligatory, whoever wanted could fast and whoever wanted could break the fast," in Sahih Bukhari 2: 704, and Sahih Muslim 2: 794. Fasting on the Day of Ashura was obligatory in Islam, then its obligation was abrogated by the fasting of Ramadan, and the Prophet, peace be upon him, gave Muslims the choice to fast it. Al-Hakam ibn Al-A'raj, may Allah be pleased with him, said: "I reached Ibn Abbas, may Allah be pleased with them, while he was resting his head on his cloak at Zamzam, and I said to him: Inform me about fasting on Ashura. He said: When you see the crescent of Muharram, then count, and fast on the ninth day. I said: Was this how the Messenger of Allah, peace be upon him, used to fast it? He said: Yes," in Sahih Muslim 2: 797, and Musnad Ahmad 1: 246.