Muharram’ is the first {1st} month of the Hijri calendar and is one of the Four [4] sacred months concerning which Allah {Aza wa Jall } says in the Quran:- ‘Surely the number of months with Allah is twelve {12} months in Allah’s ordinance since the day when He created the heavens and the earth, of these four being sacred; that is the right reckoning; therefore be not unjust to yourselves regarding them. [Source:-Quran ~Surah 9 At-Taubah, Verse # 36]

‘Muharram’ is one of the four{4} sacred Months out of twelve. The others being… Zul-Qa’dah, Zul-Hijjah, and Rajab. These four { 4 }months have been sacred since the creation of universe.

Allah {Aza wa Jall } says in the Quran:- Muharram is a sacred month because Allah made it sacred along with three{3} other months and told us about it in the Quran. Prophet Muhammad {may peace be upon him} informed us about the Four {4}sacred months in the following narration:-

‘The year is of twelve {12}months, out of which four{4} months are sacred:
Three{3} are in succession …
#1/ ~Dhul-Qa’ da.
#2/~ Dhul-Hijja.
#3/~Muharram.
#4/ ~ Rajab.
[Source:- Bukhari Vol 004, Book 054, Hadith # 419]

‘Ashura’~ [meaning tenth [10th ] in Arabic] {The 10th of Muharram}
Fasting on the 10th Muharram Ashura was obligatory on the Muslims before the fasts of Ramadan were made obligatory.

Narrated by the Mother of the Believers Aisha (Allah be pleased with her):- The people used to fast on ‘Ashura’ {the tenth (10th)day of the month of Muharram} before the fasting of Ramadan was made obligatory. And on that day the Kaaba used to be covered with a cover. When Allah made the fasting of the month of Ramadan compulsory…

Allahs Apostle {may peace be upon him} said:- ‘Whoever wishes to fast (on the day of Ashura) may do so; and whoever wishes to leave it can do so’. [Source:-‘Bukhari’ Vol 02, Book 026, Hadith # 662]

The Fast of Ashura expiates the sins of the previous year and the coming year.

Abu Qatada al-Ansari (Allah be pleased with him) reported that….. The Messenger of Allah {may peace be upon him} was asked about fasting on the day of Ashura {10th of Muharram} whereupon he said:- It expiates the sins of the preceding year. {Source:-Muslim: Book 06: Hadith # 2603}