In a Hadith narrated by Imam Muslim, the Holy Prophet Hazrat Muhammad (saww) said:
أَفْضَلُ الصِّيامِ، بَعْدَ رَمَضانَ، شَهْرُ اللهِ المُحَرَّمُ
“The best of fasts after the month of Ramadan is in the Month of Allah, which you call Muharram”
The day of Ashura is the 10th of Muharram. One of the greatest merits of fasting on the day of Ashura is that Allah s.w.t will expiate the sins from the previous year!
The fast of Ashura falls on the 10th day of Muharram. However, it is also encouraged for Muslims to fast a day before and after Ashura, i.e. 9th and 11th Muharram as well.
The Holy Prophet s.a.w.w said in a hadith:
صُومُوا قَبْلَهُ يَوْمًا، أَوْ بَعْدَهُ يَوْمًا
“Fast on the day before and the day after Ashura.”