Allah, the Almighty, decreed that the year to be divided into twelve months and declared four of them as sacred, gave them some certain privileges. These sacred months are Rajab, the seventh month of the Islamic lunar calendar, Dhu al-Qi‘da (the eleventh month), Dhu al-Hijja (the month of Hajj), and Muharram (the first month of the year).
Allah says, “Allah decrees that there are twelve months – ordained in Allah’s Book on the Day when He created the heavens and earth– four months of which are sacred: this is the correct calculation. Do not wrong one another in these months.” [9:36]

Abu Bakrah narrated that the Prophet (Allah’s peace and blessing be upon him) said, “Time has come back to its original state which it had when Allah created the Heavens and the Earth; the year is twelve months, four of which are sacred. Three of them are in succession; Dhu al-Qi‘da, Dhu al-Hijja and Al-Muharram, and (the fourth being) Rajab Mudar (named after the tribe of Mudar as they used to respect this month) which stands between Jumad (ath-thani) and Sha'ban.”

Some of the Shafiee scholars opined that Rajab is the best of the sacred months while others, such as Imam Al-nawawe, appointed Muharram to be the best of them.

These four months are called so as Allah forbade fighting within them and due to the graveness of violating and committing sins in them.
Ibn ‘Abbas said, “ Allah, the Almighty, choose four months and made them sacred thus committing sins during them is grave and the reward of the good deeds is great”.

Initiating a fight in the sacred months is prohibited.
Allah says “They ask you about (fighting) in the sacred month. Say, "Fighting therein is great [sin],”. On the other hand, defensive fighting is unanimously permissible whether the month is sacred or not.

During these sacred months, the reward of the good deeds is multiplied, so it is recommended to do as many good deeds as possible. Moreover, sinning is considered more serious than in other months. In his explanation of the verse “Do not wrong your souls in these months. ” [9:36]
Imam Ibn Katheer says, “sinning in the sacred months is further forbidden and the punishment is multiplied as it is multiplied in the sacred places.”
Allah says “Do not wrong one another in these months in these months. ” [9:36]