Fasting of the month of Muharram: Muharram is one of the sacred months of the year. Fasting this month is highly recommended. Muharram is one of the four sacred months.  

إِنَّ عِدَّةَ الشُّهُورِ عِندَ اللَّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللَّهِ يَوْمَ خَلَقَ السَّمَاوَات وَالأَرْضَ مِنْهَا أَرْبَعَةٌ حُرُم”

“Indeed, the number of months ordained by Allah is twelve—in Allah’s Record since the day He created the heavens and the earth—of which four are sacred…” Quran Ayah at-Taubah (Repentance) 9:36

We are recommended (NOT OBLIGATORY) to fast the day of Ashura, and better to fast the day before and the day of or the day of and the day after, not all three days. The recommendation to add a day before or a day after is said to be for the purpose of differentiating Muslims from other peoples who only fast the day of.

Abu Hurayrah narrated that the Prophet (Sallallaahu alayhi wa sallam) said:

“The best fasting after the month of Ramadaan is the month of Allaah, the Muharram” [Muslim]

Allaah‟s Messenger called this month the month of Allaah and as we know that when this type of idaafah is mentioned and it is related to the name of Allaah, then it means that object has greatness, honor and deserves our respect and veneration as is prescribed in the shariah. Like baytullah, naaqatullaah, rosuulullaah….

“Abdullaah Ibn Abbas reported: The Prophet came to Medina and saw the Jews fasting on the day of Aashuura. He asked them about that. They replied, “This is a good day, the day on which Allah rescued Bani Israel from their enemy. So, Moses fasted this day.” The Prophet said, “We have more claim over Moses than you.” So, the Prophet fasted on that day and ordered (the Muslims) to fast (on that day).” [Bukhari]

So, the Jews used to proclaim this day as “Yawmun Saleh” [i.e. noble day] “Yawmun Adheem” [i.e. a greayt day] and other narrations, Musa alayhis-salam was made victorious and Fer‟aun was drowned, so Musa alayhis-salam and his followers used to fast on this day.

So, we understand the great importance of this month, however fasting in this month shouldn’t be done as a whole as we know from the sunnah that the Prophet (saw) never fasted a full whole month except the month of Ramadaan. So, what the hadith in Muslim means is to fast a lot of days in Muharram – but not the whole month and Allaah knows best.

The 10th of Muharam, Aashuura: When the issue of Muharram comes, we remember a blessed day on that month and that is the day ofAashuura, the 10th of Muharram.

From the sunnah we know the following regarding, Ashuura:

• Ashura was the first obligatory fasting. But when Ramadan was made obligatory, `Aashura became optional.
• Ashura is one of the day of the days of Allaah.
• The Quraish used to fast this day in Mekkah.
• The Prophet (Sallallaahu alayhi wa sallam) used to fast this day even in Mecca.
• Musa, Alayhis-salaam was saved on this day, he and followers used to fast this day in thanksgiving to Allaah.
• Jews took this day as fasting and, Eid.
• When the Prophet (Sallallaahu alayhi wa sallam) came to Al-Medinah, he ordered the companions to fast this day but not to take this day as an „eid like the Jews did.
• The Prophet (Sallallaahu alayhi wa sallam) wished to fast the 9th of Muharram to make it different from the Jews, however before the coming year he (Sallallaahu „alayhi wa sallam) passed away.
• The Prophet (Sallallaahu alayhi wa sallam) wished to fast the 9th of Muharram fearing he might miss the Aashuura.
• Fasting the day of Ashuura will expiate the sins of the past year.

– Source: https://www.masjidibrahim.org/