"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. That is the correct religion, so do not wrong
yourselves during them..." [Quran 9:36]
Ibn
Abbaas (ra) said that this phrase (so do not wrong yourselves during
them… ) 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.The virtue of observing supererogatory fasts during Muharram
The Messenger of Allah (saws) said, "The best of fasting after Ramadan is fasting Allah's month of Muharram." [Muslim]
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-Qaari said, “The apparent meaning is all of the month of Muharram.” But it is known that the Prophet (saws) never fasted any whole month apart from Ramadan, so this Hadeeth is probably meant to encourage increasing one's fasting during Muharram, without implying that one should fast for the entire month.
It was reported that the Prophet (saws) used to fast more in Sha'baan. It is likely that the virtue of Muharram was not revealed to him until the end of his life. [An-Nawawi and Muslim]
Al-Izz ibn Abdus-Salaam said that Allah makes certain times and placed sacred or blessed in order to encourage his servants for righteous deeds and bestow a greater reward for them. [Qawaa'id Al-Ahkaam]