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 Hadrath Ibn Abbas{raziallaahu ta’aalaa anhu}, The Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam}, 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 {alaihis salaam} and his followers crossed the Red Sea miraculously and the Fir’aun was drowned in its waters. On hearing this from the Jews, the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} said, "We are more closely rotated to Musa {alaihis salaam}, than you," and directed the Muslims to fast on the day of 'Ashura'. {Abu Dawood Shariif}.
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 {raziallaahu ta’aalaa anha} has said:
"When the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} 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 Dawood Shariif}
However, the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} used to fast on the day of 'Ashura' even after the fasting in Ramadan was made obligatory. Hadrath Abdullah ibn Musa {raziallaahu ta’aalaa anhu} reports that the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} preferred the fast of 'Ashura' on the fasts of other days and preferred the fasts of Ramadhaan on the fast of 'Ashura'. {Bukhari Shariif, Muslim Shariif}
In short, it is established through a number of authentic ahadith ash-shariif that fasting on the day of 'Ashura' is Sunnah of the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} and makes one entitled to a great reward.
According to another Hadith ash-shariif, 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 {sallallaahu ta’aalaa alaihi wa’sallam} is that the Jews used to fast on the day of Ashura alone, and the Holy Prophet {sallallaahu ta’aalaa alaihi wa’sallam} 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 ash-shariif. Yet, some Scholars like Hadrath Baihaqi and Hadrath Ibn Hibban {rahmatullahi ta’aalaa alaihi} have accepted them as reliable.
What is mentioned above is all that is supported through authentic sources about Ashura.
