إِنَّ عِدَّةَ الشُّهُورِ عِنْدَ اللَّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللَّهِ يَوْمَ خَلَقَ السَّمَاوَاتِ وَالْأَرْضَ مِنْهَا أَرْبَعَةٌ حُرُمٌ ذَلِكَ الدِّينُ الْقَيِّمُ فَلَا تَظْلِمُوا فِيهِنَّ أَنْفُسَكُمْ وَقَاتِلُوا الْمُشْرِكِينَ كَافَّةً كَمَا يُقَاتِلُونَكُمْ كَافَّةً وَاعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ ﴿۳۶﴾ إِنَّمَا النَّسِيءُ زِيَادَةٌ فِي الْكُفْرِ يُضَلُّ بِهِ الَّذِينَ كَفَرُوا يُحِلُّونَهُ عَامًا وَيُحَرِّمُونَهُ عَامًا لِيُوَاطِئُوا عِدَّةَ مَا حَرَّمَ اللَّهُ فَيُحِلُّوا مَا حَرَّمَ اللَّهُ زُيِّنَ لَهُمْ سُوءُ أَعْمَالِهِمْ وَاللَّهُ لَا يَهْدِي الْقَوْمَ الْكَافِرِينَ ﴿۳۷﴾

﴾36﴿ Inna 'iddatash shuhoori 'indal laahis naa 'ashara shahran fee Kitaabil laahi yawma khalaqas samaawaati wal arda minhaaa arba'atun hurum; zaalikad deenul qaiyim; falaa tazlimoo feehinna anfusakum; wa qaatilul mushrikeena kaaaffatan kamaa yuqaati loonakum kaaaffah; wa'lamooo annal laaha ma'al muttaqeen
﴾37﴿ Inna man naseee'u ziyaadatun filkufri yudallu bihil lazeena kafaroo yuhil loonahoo 'aamanw wa yuhar rimoonahoo 'aamal liyuwaati'oo 'iddata maa harramal laahu fayuhilloo maa harramal-laah; zuyyina lahum sooo'u a'maalihim; wallaahu laa yahdil qawmal kaafireen

﴾36﴿ Indeed, the number of months with Allah Almighty is twelve months, according to Allah Almighty's decree from the day He created the heavens and the earth, of which four are sacred months This is the firm religion, so do not wrong each other in these months, and fight the polytheists all together, as they fight you all together And know that Allah, the Exalted, is with the righteous
﴾37﴿ It is nothing but that delaying (on the sacred month) is an excess in disbelief, and thereby the disbelievers are further misguided They consider that month permissible one year and forbidden the next year, in order to complete the number of months that Allah Almighty has honored So they make lawful (in this way) the months which Allah Almighty has made forbiden Their evil deeds have been made fair-seeming to them, and Allah Almighty does not grant success to the disbelieving people

[36] In this noble verse, there is also a refutation of the People of the Book, who would add a few extra days to the twelve months in a year. In some years, they even had thirteen months. This caused disruption to the timing of Hajj and Ramadan. Likewise, it is a refutation of the Arab polytheists who would alter the sacred months. The People of the Book agreed with them in this practice. “Ad-dīn al-qayyim” — according to Qurtubi, the correct interpretation is that "dīn" means the well-known religion, and qayyim means straight and upright, because this is the religion of Ibrahim (peace be upon him). In this verse, the command to fight all polytheists is repeated. "So do not wrong yourselves therein" — this means: do not commit injustice in any of these sacred months, whether by increasing the number of months, changing them, bringing them forward, delaying them, or committing other sins.
Note1:: This verse proves that in the rulings of Shari‘ah, the calculation is based on the lunar months, not the solar calendar.
Note 2: According to the lunar calendar, the total number of days in a year is 350, while in the solar calendar it is 365 and a quarter days. So, the solar year is approximately 10 days longer than the lunar year. Therefore, every three years, the difference adds up to about one month, making a 13-month year. This goes against the system of Allah the Exalted.
Note 3: The word (kāffatan) means either all types of polytheists, or all the months—based on the latter meaning, this verse abrogates the prohibition
[37] In this verse, there is a refutation of one of the false customs of the polytheists related to the months—just as the previous verse refuted the People of the Book regarding the addition of months. The detail is that three sacred months come consecutively: Dhul-Qa‘dah, Dhul-Hijjah, and Muharram. It was difficult for the Arab polytheists to refrain from fighting for three straight months. So when they returned from Mina after Hajj, a man from the tribe of Banu Kinana—named Qalammas—would stand up. No one could reject his decision. They would say to him, “Postpone the sanctity of Muharram for us.” So he would declare that the sanctity of Muharram was moved to the month of Safar, making the sacred month permissible for fighting. The following year, they would move the sanctity to Rabi‘ al-Awwal, and so on—each year shifting it to another month. The disbelief in this custom was that they would make something prohibited by Allah into something permissible. That’s why in this verse, this act is referred to as an increase in disbelief—meaning that they already had other forms of disbelief, but inventing prohibitions and permissions on their own added another layer of disbelief. Just as faith can increase, so can disbelief. "They are led astray thereby—those who disbelieve" — this misguidance comes through the whispering of Shaytan, whether human or jinn. "To adjust the count" — meaning they only consider the number of months and change the description of which months are sacred, without caring about the real sanctity of those months.