Allah’s sacred month of Muharram is a blessed and important month. It is the first month of the Hijri calendar and is one of the four sacred months concerning which Allah says (interpretation of the meaning):
The First Month of the Year – Why Is It Called Muharram?
Verily, the number of months with Allah is twelve months (in a year), so it was ordained by Allah on the Day when He created the heavens and the earth; of them, four are sacred. That is the right religion, so wrong not yourselves therein. (9:36)
Abu Bakarah reported that the Prophet (peace be upon him) said:
The year is twelve months of which four are sacred, the three consecutive months of Dhul-Qa’dah, Dhul-Hijjah and Muharram, and Rajab Mudar which comes between Jumadah and Sha’ban. (Al-Bukhari, 2958)
Muharram is so called because it is a sacred (muharram) month and to confirm its sanctity. Allah’s words (interpretation of the meaning): {… so wrong not yourselves therein …} mean do not wrong yourselves in these sacred months, because sin in these months is worse than in other months.
It was reported that Ibn ‘Abbas said that this phrase referred to all the months, then these four were singled out and made sacred, so that sin in these months is more serious and good deeds bring a greater reward.
Qatadah said concerning this phrase that wrongdoing during the sacred months is more serious and more sinful that wrongdoing at any other time.
Wrongdoing at any time is a serious matter, but Allah gives more weight to whichever of His commands He will.
Allah has chosen certain ones of His creation. He has chosen from among the angels Messengers and from among mankind Messengers.
He chose from among speech the remembrance of Him (dhikr). He chose from among the earth the mosques, from among the months Ramadan and the sacred months, from among the days Friday and from among the nights Laylatul-Qadr, so venerate that which Allah has told us to venerate.
People of understanding and wisdom venerate the things that Allah has told us to venerate. (Summarized from the Tafsir of Ibn Kathir. Tafsir of Surah at-Tawbah, ayah 36)
The Virtue of Observing More Nafil Fasts During Muharram
Abu Hurayrah said:
The Messenger of Allah (peace and blessings of Allah be upon him) said:
The best of fasting after Ramadan is fasting Allah’s month of Muharram. (Muslim, 1982)
The phrase “Allah’s month”, connecting the name of the month to the name of Allah in a genitive grammatical structure, signifies the importance of the month.
Al-Qari said:
“The apparent meaning is all of the month of Muharram.”
But it was proven that the Prophet (peace be upon him) never fasted any whole month apart from Ramadan, so this hadith is probably meant to encourage increasing one’s fasting during Muharram, without meaning that one should fast for the entire month.
It was reported that the Prophet used to fast more in Sha’ban. It is likely that the virtue of Muharram was not revealed to him until the end of his life, before he was able to fast during this month. (Sharh an-Nawawi ‘ala Sahih Muslim)
Allah Chooses Whatever Times and Places He Wills
Al-‘Izz ibn ‘Abdus-Salam said:
“Times and places may be given preferred status in two ways, either temporal or religious/spiritual.
With regard to the latter, this is because Allah bestows His generosity on His slaves at those times or in those places, by giving a greater reward for deeds done, such as giving a greater reward for fasting in Ramadan than for fasting at all other times, and also on the day of ‘Ashura’, the virtue of which is due to Allah’s generosity and kindness towards His slaves on that day … ” (Qawa’id al-Ahkam, 1/38)
Qawa’id al-Ahkam
SOURCE: https://aboutislam.net/reading-islam/understanding-islam/the-first-month-of-the-year-why-its-called-muharram/