The tenth day of this sacred month, Muharram is known as Aashoorah. It is one of the most important and blessed days of Allah in the Islaamic calendar. Some ulamaa (scholars) are of the opinion that before the fasts of Ramadhaan were made compulsory, the fast of the day of Aashoorah was compulsory upon the ummah. This is stated in a hadeeth reported by Aaishah radhiyallahu anha that the Holy Prophet sallallahu alayhi wasallam ordered the observance of the fast of Aashoorah. However, when the fast of Ramadhaan became compulsory, then whosoever wished, kept this fast and whosoever desired did not observe this fast. Bukhari Vol.1 Page 268

But, nevertheless the Prophet sallallahu alayhi wasallam continued to fast this day and encouraged his Companions to do the same. Ibne Abbas radhiyallahu anhu says: I did not see Rasulullah sallallahu alayhi wasallam anxiously await the fast of any day, which he gave preference to over other days, but this day, the day of Aashoorah. Bukhari Vol.1 Page 268

Humayd Ibne Abdur Rahamaan narrates that he heard Mu’aawiyah Ibne Abi Sufyaan radhiyallahu anhu on the day of 'Aashoorah, during the year he performed hajj, saying on the pulpit: "O the people of Madinah! Where are your ulamaa (scholars)? I heard Rasullullah sallallahu alayhi wasallam saying: This is the day of 'Aashoorah. Allah has not enjoined its fasting on you but I am fasting it. Whosoever wish, keep (this) fast and whosoever desires do not observe (this fast)." Bukhari Vol.1 Page 268

In another hadeeth, Ibne Abbas raahiyallahu anhu narrates that the Holy Prophet sallallaha alayhi wasallam came to Madinah and found the Jews fasting on the day of 'Aashoorah. Hence the Holy Prophet sallallahu alayhi wasallam inquired of them, "What is (the significance of) this day on which you fast?" They replied, "This is a great day. On this day Allah saved Moosa and his people and drowned Fir’awn and his nation. Thus Moosa alayhis salaam fasted on this day as a token of thanksgiving, therefore we too fast on this day." The Messenger of Allah sallallahu alayhi wasallam said, "We are more worthy of Moosa and nearer to him than you." Thereafter, the Holy Prophet sallallahu alayhi wasallam fasted on this day and ordered (his Companions) that a fast be kept on this day. (Muslim Vol.1 Page 359)