Allah Almighty preferred some months over others,  of these are the sacred months which have been stated in the Qur`an in general, where it reads: (Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred), At-Tawbah, 36.

The prophet Muhammad has detailed these sacred months in his sunnah where Bukhari and Muslim have reported that the Messenger of Allah says: (Indeed, time has changed to the same state when Allah has created the heavens and earth; a year consists of twelve months, from which four are sacred; three are successive: Dhul-Qi`adah, Dhul-Hijjah, Muharram, and Rajab that called Mudar, which comes between  Jumada and Sha`ban.)

Many narrataions exhorts fasting alot in these sacred months, however, they are weak, except  for what have been narrated in connection to fasting the month of Muharram, since fasting in it is a stressed sunnah, where Muslim related on the authority of Abu-Hurairah that the Prophet said: (The best fasting after Ramadan is fasting in Muharram, and the best prayer after the obligatory one is night prayer.)

Accordingly, let's numerate days in Muharram to fast, and we would keep in mind that fasting one day distances the fasting Musim from hell-fire on the Day of Resurrection as the Prophet said: (A muslim fasting one day in the cause of Allah will be distanced seventy years from hell-fire.)

http://300035.dotdothktrade.tech/en/artical/3407665/sunnah-fasting-muharram