Why Is Rajab Separate from Other Sacred Months?
Question

Why is the month of Rajab separate from the other sacred months?
Praise be to Allah.Firstly:
What are the sacred months?
Allah, may He be exalted, says (interpretation of the meaning):
{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. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him]} [at-Tawbah 9:36].
The sacred months are: Rajab of Mudar, and three consecutive months: Dhu’l-Qa‘dah, Dhu’l-Hijjah and Muharram, as is stated in many reports from the Messenger of Allah (blessings and peace of Allah be upon him). See: Tafseer at-Tabari (11/440).
Al-Bukhaari (4406) and Muslim (1679) narrated from Abu Bakrah, that the Prophet (blessings and peace of Allah be upon him) said: “Time has returned to its original order as it was on the day when Allah created the heavens and the earth. The year is twelve months, of which four are sacred: three consecutive months, Dhu’l-Qa‘dah, Dhu’l-Hijjah and Muharram, and Rajab of Mudar, which comes between Jumaada and Sha‘baan.”
Al-Waahidi said in al-Baseet (10/409): What is meant by sacred is that it is a very serious matter to transgress the sacred limits in these months, in a manner that is more serious than doing so at other times. The Arabs used to venerate [the sacred months] to the extent that if a man met his father’s killer, he would not disturb him.
The scholars said: There are some benefits in making some months more sacred than others, such as the fact that there is an interest to be served when people refrain from committing transgressions and evil deeds in the sacred months, because Allah gave them a high status, for perhaps that might lead to giving up wrongdoing completely, as the motive for such actions may diminish during that period. End quote.
Secondly:
The wisdom behind distinguishing the sacred months from other months
Regarding the distinguishing of these months from others, this is something that Allah has not told us [about the reason for it]. It is like other commands that Allah has enjoined upon us, so we must comply in the manner that Allah has enjoined, even if we do not understand the wisdom behind the command. We must realise that Allah, may He be and glorified, is wise in what He enjoins and forbids, although that should not prevent us from seeking to find an explanation to the wisdom behind it.
The reason why Rajab is separate from the other sacred months
Some of the scholars have stated that the reason why Rajab is separate from the other sacred months is that it was to enable the Arabs to do ‘umrah in the middle of the year, and the consecutive sacred months were so that they could do Hajj.
Ibn Katheer (may Allah have mercy on him) said:
{of these, four are sacred} – these were months that the Arabs regarded as sacred during the Jaahiliyyah. This is what was followed by the majority, apart from a group among them who were called al-busl; they regarded eight months of the year as sacred, by way of strictness and going to extremes.
Regarding the words [in the hadith] “three consecutive months: Dhu’l-Qa‘dah, Dhu’l-Hijjah and Muharram, and Rajab of Mudar, which comes between Jumaada and Sha‘baan,” Rajab is attributed to [the tribe of] Mudar to confirm the soundness of their view that Rajab is the month between Jumaada and Sha‘baan, and it was not as [the tribe of] Rabee‘ah thought, that “Rajab al-Muharraam” was the month between Sha‘baan and Shawwaal, which is known as Ramadan today. Thus the Prophet (blessings and peace of Allah be upon him) explained that it was the Rajab of Mudar, not the Rajab of Rabee‘ah.
The sacred months were four, three consecutive and one on its own, for the purpose of performing the rituals of Hajj and ‘umrah. So the month before the month of Hajj was made sacred, namely Dhu’l-Qa‘dah, because they refrained (yaq‘udoona) from fighting during that month. The month of Dhu’l-Hijjah was made sacred, because in that month they would do Hajj and focus on performing the rituals. And another month after that was made sacred, namely Muharram, so that they could return to their distant lands safely.
Rajab, in the middle of the year, was made sacred so that people could go and visit the Ka‘bah and do ‘umrah, for those who came to it from the furthest parts of the Arabian peninsula to visit the Ka‘bah then return safely to their homelands.
The words {That is the correct religion} mean: this is the straight path of complying with the command of Allah regarding what He has designated as the sacred months and so that they might conduct themselves during these months in accordance with the previous book of Allah.
The words  so do not wrong yourselves during them} mean: during the sacred months, because sin during these months is more serious and more abhorrent than in other months, just as the burden of sin committed in the sacred land is multiplied. End quote from at-Tafseer (4/148).
Ar-Raazi said in at-Tafseer (16/41):
If it is said: in reality, different units and divisions of time are similar in nature, so what is the reason for this singling out of certain times?
We say: This singling out of certain times is not something strange in different religious laws, for there are many examples of that. Do you not see that Allah, may He be exalted, singled out the sacred land from other lands by making it more sacred; and He singled out Friday from other days of the week by making it more sacred; and He singled out the day of ‘Arafah from other days for this specific act of worship; and He singled out the month of Ramadan from other months by making it more sacred by enjoining the fast; and He singled out some hours of the day by making it obligatory to pray at those times; and He singled out one night from all others, namely Laylat al-Qadr; and He singled out some people from all other people by giving them the gift of prophethood?
As these examples are clear and well known, why is it strange that some months should be singled out by being made more sacred? Moreover, it is not far-fetched to say that Allah knows that certain acts of worship are more effective in purifying the soul at these times, and committing sin at these times has a greater impact of contaminating the soul. This is not far-fetched for wise people. Do you not see that some scholars have written books about the times when it is more likely that supplications will be answered, and they stated that in those specific times there are factors which dictate that supplications should be answered.
… We also learn something else from it, which is that people have a natural inclination towards committing wrongdoing and spreading mischief, and for them to refrain completely from these bad deeds is very difficult. So Allah, may He be glorified and exalted, has singled out certain times to be venerated and respected, and He has singled out certain places to be venerated and respected, so that if a person refrains from sin and bad deeds at those times and in those places, that will lead to a number of virtues and benefits:
1.. Refraining from evil deeds at those times is something desirable, because it reduces evil deeds.
2.. When a person gives up evil deeds at those times, perhaps giving them up at those times will encourage him to give them up at all times.
3.. If a person does acts of worship at those times and keeps away from sin, then after those times end, if he goes back to evil deeds and sin, going back to them will be a cause of all the trouble that he went to of doing those acts of worship at those times being rendered worthless. Obviously a man of sound reasoning will not accept that, so that may lead to him giving up sin altogether.
This is the wisdom behind the singling out of certain times and places for more veneration and respect. End quote.
Some of the scholars stated that this comes under the heading of universal issues, the wisdom behind which cannot be asked about, even though there are benefits in them.
At-Taahir said: You should understand that giving precedence to certain times and places is like giving precedence to certain people. People may be given precedence because of what they do of righteous deeds and what they have of noble characteristics. Giving precedence to other things (times and places) is because of what is done in them of virtuous deeds.
Giving precedence to certain times and places is something that is decided by Allah, may He be exalted, and He has told us about it, or He has told us what He wants from us, because when He gives precedence to it, he makes it a time or place in which His pleasure may be sought, and a time or place in which supplication is likely to be answered, or the reward for good deeds is likely to be multiplied, as He, may He be exalted, says (interpretation of the meaning): {The Night of Decree is better than a thousand months} [al-Qadr 97:3]. In other words, it is better than the worship of a thousand months for those nations who came before us. The Prophet (blessings and peace of Allah be upon him) said: “One prayer in this mosque of mine is better than a thousand prayers offered anywhere else except al-Masjid al-Haraam.”
Allah knows best the wisdom behind giving precedence to one time over another, or to one place over another. The things that are decided by Allah, may He be exalted, are affairs and situations that He has willed and decreed. Thus they are similar to universal issues, so they cannot be invalidated unless Allah Himself invalidates them, as He cancelled the veneration of the sabbath, which He replaced with Friday (Jumu‘ah).
People have no right to decide about religious times, because the matters that people decide about are like things that they make with their hands, and they have no value unless they are intended to serve a good purpose. Thus people have no right to change what Allah, may He be exalted, has decreed of virtue for certain times, places or people. End quote from at-Tahreer wa’t-Tanweer (10/184).
And Allah knows best.