Listing the four sacred months in his final sermon (khuṭbat al-wadāʿ), the Prophet explains: ‘Four of the months are sacred [ḥurum], three of which are sequential – Dhu al-Qaʿda, Dhu al-Hijja, and Muharram, and Rajab Mudar [which falls] between Jumada and Shaʿban’. Some interpreters regarded this seemingly extraneous detail concerning Rajab's alignment as a reference to the abolishment of the pre-Islamic practice of intercalation (ib ṭālan li-l-nasīʾ alladhī kānat al-ʿArab yafʿalūhu fī al-Jāhiliyya).

According to the Qurʾan, intercalation was one of the pagan Arabs’ misdeeds:

According to Muslim exegetes, the verse accuses the pagans of Arabia of adding a month, the nasīʾ, roughly once every three years, so as to postpone the beginning of the sacred months and delay ‘God's peace’ for their own convenience. As a result, they also observed the hajj on the wrong days, rendering the sacred profane and the profane sacred.

Al-Biruni, who wrote a comparative essay on calendars in the first half of the eleventh century, postulates that the Arabs learned intercalation (kabs) from Jewish people in the area, ‘200 years before the hijra’– that is, in the fifth century CE. Other medieval Muslim scholars stressed that the abolition of the leap year restored the divinely prescribed ‘natural order’ and ensured the timely observation of sacred rites. Ibn Taymiyya claims that the abrogation of intercalation restored the correct ‘Abrahamic’ calendar. He especially refers to the securing of the proper timing of the hajj – which even Abu Bakr erroneously performed during Dhu al-Qaʿda (rather than on Dhu al-Hijja) in the ninth year of the hijra – for years to come.

Ibn Taymiyya points to the moon as a divinely appointed indicator of the months and years, citing the Qurʾan: ‘It is He who made the sun a shining light and the moon a derived light and determined for it phases, that you may know the number of years and account [of time]’ (Q. 10: 5).