Assalamualaikum wrh wbht
MUHARRAM is the first month of the Islmc, Hijrah calendar. It is one of the four sanctified months about which Allah Ta'ala says, 'The number of months in the sight of Allah are twelve (in a year) - So ordained by Him the day He created the heavens and the earth; Of them four are sacred: That is the right religion. So wrong not yourselves therein.' (Surah Taubah).
The four months, according to authentic Ahaadith, are the months of Zul-Qa`dah, Zul Hijjah, Muharram and Rajab. The sanctity of these four months was observed in the Shari'ah of all the Ambiyaa (AS).
'Allah Ta'ala made them (i.e. the four months) sacred and increased their sanctity. Hence declared a sin therein more severe and increased the virtue of good deeds and (also) the reward thereof.' (Ibn Katheer).
THE MONTH OF MUHARRAM
The best of fasts besides the month of Ramadhaan is the fast during Allah's month of Muharram and the best of Salaats besides the Fardh (obligatory Salaat) is the Tahajjud Salaah (performed after midnight). (Sahih Muslm).
THE DAY OF AASHURA (10th OF MUHARRAM)
The tenth of Muharram, known as the day of Aashura, is a very significant day in the Islmc calendar. Nabi (SA) has exhorted the Ummah to fast on this day. Hadhrat Aaisha (RA) reports that Nabi (SAW) ordered the observance of the fast of Aashura. However, when the fast of Ramadhaan became compulsory, Nabi (SAW) left the fast of Aashura (i.e. the compulsion). Thereafter, whosoever desired kept this fast and whosoever desired, (not to keep it) did not observe this fast'. (Sahih Bukhari, Vol 1).
Hadhrat Ibn Abbaas (RA) says, 'I did not see Rasulullah (SAW) anxiously await the fast of any day, which he gave preference to over other days, but this day (of Aashura) and the month of Ramadhaan (Sahih Bukhaari, Vol. 1).
VIRTUE OF THE FAST OF AASHURA
With regard to the blessing of this fast, Nabi (SAW) was asked regarding it's virtue. He answered, 'It is a compansation for the sins of the past year.' i.e. the sins of one year that has just gone by will be forgiven. (Sahih Muslim, Vol. 1). Note: Only minor sins are forgiven. For the forgiveness of major sins, Towbah is a prerequisite.
THE FAST OF AASHURA
Hadhrat Ibn Abbaas (RA) narrates that when Nabi (SAW) came to Madinah and found the Je-ws fasting on the day of Aashura, he inquired, 'What is the significance of this day on which you fast?' They replied, 'This is the day Allah saved Nabi Moosa (AS) and his followers and drowned Fir'oun and his army, thus Moosa (AS) fasted on this day as a token of thanksgiving. Therefore we too fast on this day.' (Sahih Muslim, Vol 1). Rasulullah (SAW) said, 'We are more worthy of Moosa (AS) and nearer to him that you.' Thereafter, he fasted on this day and ordered the Sahaabah to fast on this day. (Sahih Mslm, Vol. 1)
Rasulullah (SAW) is reported to have said, 'Observe the fast of Aashura and oppose (the way of) the Jews. Fast a day before it (also) or a day after it.' (Baihaqi, Vol 4)
Hence, one should fast for two days: Either the 9th and 10th or the 10th and 11th. To fast only on the 10th of Muharram is MAKROOH (reprehensible) and bears semblance to the Je-ws.