In Pre-Islamic Arabia, a lunisolar calendar was in use with twelve regular months and an occasional intercalary month.
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Intercalation
The term for the intercalary month was an-nasīʾ (النسيء). Various interpretations of this word have been given, among them a "postponing". Some sources say that the Arabs followed the Jewish practice and intercalated seven months over nineteen years, or else that they intercalated nine months over 24 years; there is, however, no consensus among scholars on this issue. The Kinānah tribe, during the time of Muhammad, was in charge of authorizing the intercalation; that the Kinānah tribe had taken over this task from the Kindah tribe, which had been Judaized for hundreds of years previously, lends credence to the position that the process of intercalation may have been borrowed from the Jewish tradition. Referring to Abū Rayḥān al-Bīrūnī (d. ca. 442 A.H./1050 C.E.), it has been posited that this intercalation was effected in order to accommodate the scheduling of seasonal trade cycles with annual pilgrimages, as such pilgrimages were occurring at least two hundred years prior to the advent of Islam.
The prohibition of intercalation in AH 10 has been suggested as having had the purpose of wresting power from the Kinānah clan who was in control of intercalation, but there is no consensus regarding this position.
Pre-Islamic month names
Sources for the names of these pre-Islamic months are Al-muntakhab min gharīb kalām alʿarab of Abū al-ḥasan ʿalī bin al-ḥasan bin al-ḥusayn al-hunāʾī ad-dūsā (d. 309 A.H./921 C.E.)(known more commonly as "Kurāʿ an-naml") and Lisān al-ʿarab of Ibn Manẓūr (d. 711 A.H./1311 C.E.).