The Quran

إِنَّ عِدَّةَ الشُّهورِ عِندَ اللَّهِ اثنا عَشَرَ شَهرًا في كِتابِ اللَّهِ يَومَ خَلَقَ السَّماواتِ وَالأَرضَ مِنها أَربَعَةٌ حُرُمٌ ۚ ذٰلِكَ الدّينُ القَيِّمُ ۚ فَلا تَظلِموا فيهِنَّ أَنفُسَكُم ۚ وَقاتِلُوا المُشرِكينَ كافَّةً كَما يُقاتِلونَكُم كافَّةً ۚ وَاعلَموا أَنَّ اللَّهَ مَعَ المُتَّقينَ

Muhammad Asad

BEHOLD, the number of months, in the sight of God, is twelve months, [laid down] in God's decree on the day when He created the heavens and the earth; [and] out of these, four are sacred:¹ this is the ever-true law [of God]. Do not, then, sin against² yourselves with regard to these [months]. And fight against those who ascribe divinity to aught beside God, all together - just as they fight against you, [O believers,] all together³ - and know that God is with those who are cconscious of Him.

This connects with the subsequent reference to fighting against "those who ascribe divinity to aught beside God" (see next note). The months spoken of here are lunar months, progressively rotating through the seasons of the solar year (see surah 2, note 165). Since reckoning by the easily observable lunar months is more natural than by the arbitrarily fixed months of the solar year, it is described in this passage as "the ever-true law (din) [of God]". The four "sacred months" during which warfare was considered blasphemous in pre-Islamic Arabia - a view which has been confirmed by Islam (see note 6 above) - are Muharram, Rajab, Dhu'l-Qa'dah and Dhu'l-Hijjah.
In their endeavour to obviate certain disadvantages for their trade caused by the seasonal rotation of the lunar months, the pagan Arabs used to intercalate a thirteenth month in the third, sixth and eighth year of every eight-year period, with a view to making the lunar calendar more or less stationary, and thus roughly corresponding to the solar year. An acceptance of this unwarranted intercalation by the Muslims would have tied the-Mecca pilgrimage as well as the fast of Ramadan to fixed seasons, and would thus have made, permanently, the performance of these religious duties either too exacting or too easy; and in either case the believers would have been offending against the spiritual purpose underlying these duties - which is the meaning of the words "do not sin against yourselves with regard to these [months]": i.e., by following, without any warrant from God, a custom devised by "those who ascribe divinity to aught beside Him", to whom the sequence refers.
I.e., "just as all of them are, as it were, united against you in their rejection of the truth, be united against them in your readiness for self-sacrifice". As regards the circumstances in which the Muslims are authorized to make war against unbelievers, see the earlier parts of this surah, and especially verses 12-13, as well as 2:190-194, where the general principles relating to war are laid down.