إِنَّ عِدَّةَ الشُّهُورِ عِندَ اللَّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللَّهِ يَوْمَ خَلَقَ السَّمَاوَاتِ وَالْأَرْضَ مِنْهَا أَرْبَعَةٌ حُرُمٌ ۚ ذَٰلِكَ الدِّينُ الْقَيِّمُ ۚ فَلَا تَظْلِمُوا فِيهِنَّ أَنفُسَكُمْ ۚ وَقَاتِلُوا الْمُشْرِكِينَ كَافَّةً كَمَا يُقَاتِلُونَكُمْ كَافَّةً ۚ وَاعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ (36)
(9:36) The fact is that the number of months ordained by Allah has been twelve since the time He created the heavens and the earth, *34 and out of these four are forbidden months: this is the right code of reckoning: therefore do not wrong yourselves by violating these months. *35 And fight against the mushriks all together even as they fight against you all together: *36 and note it well that Allah is with the Godfearing people.
*34) This means that ever since Allah created the sun, the moon and the earth, the new moon takes place only once in a month; thus the year has always been of twelve months.
This has been mentioned to refute the practice of nasi (v. 37) whereby the pagan Arabs increased the number of the months of a year to 13 or 14 to enable them to interpose in the calendar the forbidden month which had been made lawful by them. (For fuller explanation see E.N. 37).
*35) "....do not wrong yourselves ....": "Do not let go waste the good things for which fighting has been made unlawful during these months by creating disorder in them that will ultimately be harmful to you also" . The four forbidden months are . . Zil-Q`adah, Zil-Hajjah and Muharram for Haj and Rajab for ' Umrah.
*36) That is, 'If the mushriks do not desist from fighting during these months, you should also fight them and present a united front against them, just as they put up a united front against you". This verse has been explained by v. 194 of Al-Baqarah.