u/Main-Guidance-6767

Fasting on the Day of Ashoorah is a Sunnah Mu-akkadah [i.e. a practice the Prophet (sallal-laahu-alayhi-wasallam) performed continuously).  Therefore, the Muslim fasts on the 9th of Muharram and the 10th of Muharram, and this is a Sunnah that has continued—all praise is due to Allaah—and fasting on this day is emphasized, whilst seeking Allaah’s reward and in order to thank Him.

Fasting the day of ‘Ashoorah’ expiates (minor) sins for the past year, because the Prophet (peace and blessings of Allaah be upon him) said: “Fasting the day of ‘Arafah I hope Allaah will expiate thereby for the year before it and the year after it, and fasting the day of ‘Ashoorah’ I hope Allaah will expiate thereby for the year that came before it.” Narrated by Muslim, 1162.

{Please note, Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: The expiation of purification, prayer, and fasting Ramadaan, ‘Arafah and ‘Ashoora’ applies to minor sins only.(Al-Fataawa al-Kubra, part 5.)}