Repentance
[9:37]
The intercalation [of months] is but one more instance of [their] refusal to acknowledge the truth – [a means] by which those who are bent on denying the truth are led astray. They declare this [intercalation] to be permissible in one year and forbidden in [another] year, in order to conform [outwardly] to the number of months which God has hallowed: and thus they make allowable what God has forbidden. Goodly seems unto them the evil of their own doings, since God does not grace with His guidance people who refuse to acknowledge the truth.

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* v.37 : Lit., “is but an increase in denying the truth (kufr).” The term nasī’, rendered by me as “intercalation,” may also be translated as “postponement” – i.e., the postponement of lunar months by means of the periodical intercalation of a thirteenth month, as practiced by the pre-Islamic Arabs with a view to bringing the traditional lunar calendar, for purely worldly reasons, into accord with the solar year (see note 54 above). The Qur’ān describes this practice as an additional instance of kufr because it contravenes God’s declared will as regards the observance of the lunar calendar in respect of various religious duties (cf. the preceding verse, as well as 2:189 and the corresponding note 165).
* An allusion to the arbitrary manner in which the pre-Islamic Arabs intercalated a thirteenth month in the third, sixth, and eighth year of every eight-year period.
* By means of the intercalation spoken of above, the pagan Arabs did in most years keep the number of months to twelve; but by divorcing the four “sacred months” (Muharram, Rajab, Dhu ’l-Qa‘dah and Dhu ’l-Hijjah), from their proper lunar context they obviously profaned and perverted the natural law.