Surah At-Taubah, Ayat 36-37

Ayat 36. 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, (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, and fight against the Mushrikûn (polytheists, pagans, idolaters, disbelievers in the Oneness of Allah) collectively , as they fight against you collectively. But know that Allah is with those who are Al-Muttaqun (the pious)

Ayat 37. The postponing (of a Sacred Month) is indeed an addition to disbelief: thereby the disbelievers are led astray, for they make it lawful one year and forbid it another year in order to adjust the number of months forbidden by Allah, and make such forbidden ones lawful. The evil of their deeds seems pleasing to them. And Allah guides not the people, who disbelieve.

Hazrat Abu Bakrah (May Allah be pleased with him) reported: The Messenger of Allah [SAWW](PBUH) said, "Time has completed its cycle and has come to the state of the day when Allah created the heavens and the earth. The year consists of twelve months of which four are inviolable; three of them consecutive - Dhul-Qa`dah, Dhul-Hijjah and Muharram and Rajab, the month of Mudar (tribe), which comes between Jumada and Shaban. What month is this?'' We said, "Allah and His Messenger [SAWW](PBUH) know better". The Prophet [SAWW](PBUH) remained silent for some time until we thought that he would give it a name other than its real name. Then asked, "Is it not (the month of) Dhul-Hijjah?''. We replied in the affirmative. [Al-Bukhari Hadith # 4406].

Lesson : as mentioned above in Surah At-Taubah Ayat 37. "The postponing (of a Sacred Month) is indeed an addition to disbelief: thereby the disbelievers are led astray, for they make it lawful one year and forbid it another year in order to adjust the number of months forbidden by Allah, and make such forbidden ones lawful". The cycle of years and months (that is to say a month consists of 30 or 29 days and a year has 12 months) was completed at the time of creation of heavens and earth.