Ashura' (i.e. the tenth of Muharram) was a day on which the tribe of Quraish used to fast in the prelslamicperiod of ignorance. The Prophet (ﷺ) also used to fast on this day. So when he migrated toMedina, he fasted on it and ordered (the Muslims) to fast on it. When the fasting of Ramadan wasenjoined, it became optional for the people to fast or not to fast on the day of Ashura.