Allah subhanahu wa ta`ala says in Surah at-Tawbah Ayah 36,
إِنَّ عِدَّةَ الشُّهُورِ عِندَ اللَّهِ اثْنَا عَشَرَ شَهْراً فِي كِتَـبِ اللَّهِ يَوْمَ خَلَقَ السَّمَوَت وَالاٌّرْضَ مِنْهَآ أَرْبَعَةٌ حُرُمٌ ذلِكَ الدِّينُ الْقَيِّمُ فَلاَ تَظْلِمُواْ فِيهِنَّ أَنفُسَكُمْ وَقَاتِلُواْ الْمُشْرِكِينَ كَآفَّةً كَمَا يُقَـتِلُونَكُمْ كَآفَّةً وَاعْلَمُواْ أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ
“Verily, the number of months with Allah is twelve months (in a year), so was it ordained by Allah on the Day when He created the heavens and the earth; of them four are sacred. That is the right religion, so wrong not yourselves therein, and fight against the Mushrikeen idolaters collectively as they fight against you collectively. But know that Allah is with those who have Taqwa.”
It is with the wisdom of Allah subhanahu wa ta`ala that He favored some months over others, like favoring the month of Ramadan over all other months, because it is the month of mercy, the month of Qur’an, the month of forgiveness and freeing of people from the Fire. It is a month which people glorified during the pre-Islamic period as well, and after Islam it became even more glorified.
The month of Muharram (which means forbidden in Arabic) was called that because the Arabs used to forbid fighting during it.
Safar (which means zero in Arabic) was given this name because the Arabs used to loot all the property of the enemy after defeating them in battle (i.e. they left nothing behind).
Rabee’ Al-Awwal (which means graze in Arabic) because they used to graze their cattle during this month.
Jumaadaa (which means solid in Arabic) was given this name because water used to freeze during this month.
Rajab (which means remove in Arabic) was given this name because the Arabs used to remove the heads of their spears and refrain from fighting.
Sha’baan (which is anything positioned between two things in Arabic) was given this name because it comes between Ramadan and Rajab.
Ramadan (which means heat in Arabic) was given this name because of the hot temperature and excessive heat of the sun during this month.
Shawwaal (which means raise in Arabic) was given this name because she-camels would raise their tails when they became pregnant.
Thul-Qi’dah (which means sitting in Arabic) was given this name because it was the month during which they would sit and stop fighting.
Thul-Hijjah (which refers to Hajj in Arabic) was given this name because it was the month during which they performed Hajj.
In the above-mentioned ayah, Allah subhanahu wa ta`ala informs us that since He created the Heavens and the earth, and created days and nights, and made the sun and the moon, each float in an orbit, as a result of which the darkness of nights and the light of days occur. It was from that time that Allah subhanahu wa ta`ala had set the months to be twelve, and He informed us that He designated four of them to be sacred.
We must respect the sanctity of these sacred months, by adhering to the commandments of Allah subhanahu wa ta`ala, and rejecting that which the people of the pre-Islamic period were upon, like delaying the restrictions of these months or changing their sequence. It is for this reason that the Prophet salAllahu `alayhi wa sallaam said in his farewell pilgrimage,
“O People! Time has gone back to how it was at the time Allah subhanahu wa ta`ala created the Heavens and the earth. A year has twelve months, four of which are sacred, three consecutive, Thul-Qi’dah, Thul-Hijjah, Muharram, and Rajab, which comes between Jumaadaa and Sha’baan.” [Al-Bukhari]
“…when He created the Heavens and earth…”[at-Tawbah 9: 36] clarify that the decree of Allah subhanahu wa ta`ala took place very early on, and that He set the months, named them and sequenced them when He created the Heavens and the earth, then He revealed this to His prophets through the Divine Books.
The ayah is informing us that the ruling of these months will remain the same (meaning their names, sequence and sacredness), and the changes made by the disbelievers to the sequence would have no effect. Moreover, whatever the disbelievers had changed could not change what Allah subhanahu wa ta`ala had decreed.
This ayah also indicates that it is compulsory to base acts of worship and Islamic rulings on these months.
They are: Thul-Qi’dah, Dhul-Hijjah, Muharram, and Rajab which is between Jumaadaa Al-Akhirah and Sha’baan. It was also called Rajab of Mudhar, because the tribe of Mudhar used to adhere to its sacredness, while another tribe by the name of Rabee’ah ibn Nizaar used to change sacredness of Rajab to Ramadan. In order to lift this confusion the Prophet salAllahu `alayhi wa sallaam said, “Rajab, which comes between Jumaadaa and Sha’baan”.
Allah subhanahu wa ta`ala says in the same ayah (which means), “…that is the right religion” meaning, this Shari’ah [Islamic law)]and obedience therein, is the right and straight path. Then Allah subhanahu wa ta`ala continues saying that (which means), “Do not wrong (oppress) yourselves therein” [at-Tawbah 9: 36] which is referring to all months of the year but the sacred months hold a special position which makes oppressing oneself greater, just as Allah subhanahu wa ta`ala says (what means), “no sexual relation (with spouse), no sinning, nor unjust dispute” [ al-Baqarah 2: 197] and this certainly does not indicate that one can sin other than the time of Hajj, but it is to reflect the greatness of sinning during its rituals.
Do not wrong (oppress) yourselves by fighting and committing sins, because when Allah subhanahu wa ta`ala honors something for one reason, it becomes that much honored, but when He honors for two or more reasons, then sacredness becomes more, and punishment for disobedience is multiplied accordingly.
For example, one who obeys Allah subhanahu wa ta`ala during the sacred months in the sacred area will receive more reward than obeying Allah subhanahu wa ta`ala during other months, and the one who obeys Allah subhanahu wa ta`ala during months other than the sacred months outside the sacred area will receive less reward than he who obeys Allah subhanahu wa ta`ala inside the sacred area.
Allah subhanahu wa ta`ala gave an indication to this in His saying (which means): “O wives of the Prophet, whoever of you should commit a clear immorality – for her the punishment will be doubled two fold, and ever is that for Allah, easy.”[al-Ahzaab 33:30]