Although Muharram is a sanctified month as a whole, yet, the 10th day of Muharram is the most sacred among all its days. The day is named `Ashurah`. According to the Holy Companion Ibn `Abbas, Radi-Allahu anhu. The Holy Prophet (Peace Be Upon Him), when migrated to Madinah, found that the Jews of Madinah used to fast on the 10th day of Muharram. They said that it was the day on which the Holy Prophet Musa (Moses), alayhis salam, and his followers crossed the Red Sea miraculously and the Pharaoh was drowned in its waters. On hearing this from the Jews, the Holy Prophet (Peace Be Upon Him), said, "We are more closely rotated to Musa, alayhi salam, than you," and directed the Muslims to fast on the day of `Ashura`. (Abu Dawood)
It is also reported in a number of authentic traditions that in the beginning, fasting on the day of `Ashura` was obligatory for the Muslims. It was later that the fasts of Ramadan were made obligatory and the fast on the day of Ashura` was made optional. Sayyidina `Aisha, Radi-Allahu anha, has said:
When the Holy Prophet (Peace Be Upon Him), came to Madinah, he fasted on the day of `Ashura` and directed the people to fast. But when the fasts of Ramadan were made obligatory, the obligation of fasting was confined to Ramadan and the obligatory nature of the fast of `Ashura` was abandoned. Whoever so desires should fast on it and any other who so likes can avoid fasting on it." (Sunan Abu Dawud)
However, the Holy Prophet (Peace Be Upon Him), used to fast on the day of `Ashura` even after the fasting in Ramadan was made obligatory. Abdullah ibn Musa, Radi-Allahu anhu, reports that the Holy Prophet (Peace Be Upon Him), preferred the fast of `Ashura` on the fasts of other days and preferred the fasts of Ramadhaan on the fast of `Ashura`. (Bukhari and Muslim)
In short, it is established through a number of authentic ahadith that fasting on the day of `Ashura` is Sunnah of the Holy Prophet (Peace Be Upon Him), and makes one entitled to a great reward.
According to another Hadith, it is more advisable that the fast of `Ashura` should either be preceded or followed by another fast. It means that one should fast two days: the 9th and 10th of Muharram or the 10th and 11th. The reason of this additional fast as mentioned by the Holy Prophet (Peace Be Upon Him), is that the Jews used to fast on the day of`Ashura alone, and the Holy Prophet (Peace Be Upon Him), wanted to distinguish the Muslim way of fasting from that of Jews. Therefore, he advised the Muslims to add another fast to that of `Ashura`.
Some traditions signify another feature of the day of `Ashura. According to these traditions, one should be more generous to his family by providing more food to them on this day as compared to other days. These traditions are not very authentic according to the science of Hadith. Yet, some Scholars like Baihaqi and Ibn Hibban have accepted them as reliable.