It is now optional but highly preferred for Muslims to fast either the 10th or 9th and 10th or the 10th and 11th or ever all three (9, 10, 11) in the month of Muharram, as this was established at the time of the prophet Muhammad, peace be ipon him.
(Quran 2:183)
"O you who believe, fasting has been ordered for you as it was ordered for those before you, that you may attain righteousness."
However, before next year Allah revealed the verses in Surah Baqarah (chapter 2) dealing with the Commandment for fasting the entire month of Ramadan;
" (the number '10' is 'ashurah' in Arabic) of Muharram, by the prophet Muhammad, peace be upon him, because he found the Children of Israel fasting on this same day. He also mentioned the next year they would fast the 9th day of Muharram as an obligation.AshurahMuslims were first ordered only to fast the day of "

Muharram is considered one of the most sacred months, except of course the month of Ramadhan.
" (forbidden) to engage in war during this month. Haram" because it is "HaramMuharram taken from the same root as the word "

) is the first month of the Islamic calendar. It is one of the four sacred months of the year in which fighting is prohibited. Since the Islamic calendar is lunar, Muharram moves from year to year when compared with the Gregorian calendar. محرم:Arabic (Muharram