"Verily, the number of months with Allaah is twelve months (in a year), so was it ordained by Allaah on the Day when He created the heavens and the earth; of them four are Sacred (i.e. the 1st, the 7th, the 11th and the 12th months of the Islamic calendar). That is the right religion, so wrong not yourselves therein" (Tawbah 9:36)
ثَلَاثَةٌ مُتَوَالِيَاتٌ: ذُو الْقَعْدَةِ وَذُو الْحِجَّةِ وَالْمُحَرَّمُ وَرَجَبُ مُضَرَ الَّذِي بَيْنَ جُمَادَى وَشَعْبَان
The Prophet said regarding the four sacred months, "Three are in succession; Dhul-Qa`dah, Dhul-Hijjah and Muharram, and (the fourth is) Rajab of (the tribe of) Mudar which comes between Jumada (Ath-Thani) and Sha`ban. (Al Bukhaari (4662) and Muslim (1679): Narrated from Abu Bakrah (may Allaah be pleased with him))
The Prophet said "Rajab of Mudar'' to attest to the custom of Mudar, in saying that Rajab is the month that is between Jumada and Sha`ban, not as the tribe of Rabi`ah thought, that it is between Sha`ban and Shawwal, which is Ramadan in the present calendar.
The four Sacred Months were made four, three in succession and one alone, so that the Hajj and `Umrah are performed with ease.
- Dhul-Qa`dah, the month before the Hajj month, was made sacred because tribes refrained from fighting during that month and people start to travel for Hajj.
- Dhul-Hijjah, the next month, was made sacred because it is the month of Hajj, during which Hajj rituals are performed.
- Muharram, which comes next, was made sacred so that people are able to go back to their areas in safety after performing Hajj.
- Rajab, in the middle of the lunar year, was made sacred so that those coming from the farthest areas of Arabia are able to perform `Umrah and visit the House of Allah and then go back to their areas safely.