Jewish roots
There have been several attempts to trace the roots of Jewish aspects of the Islamic Calendar. The Shorter Encyclopaedia of Islam says: "The starting-point for a comparison between the Islamic and the Jewish Calendar is the optional fast-day Ashura, the tenth day of Muharram, which is an Islamic continuation of the Day of Atonement”.[1] G. Widengren says: “If Muharram is comparable with Tishri, then the month of Ramadhan is comparable to the Jewish month of Sivan, the month of the Jewish Feast of Weeks. Islamic tradition lays down that that it was on the nights of Ramadhan, … Lailat Al-Kadr … that Muhammed received the revelation of the Koran. … there is a clear parallel between the circumstances in which Moses received the Torah and those in which Muhammed received the Koran”.[2]
In order to trace the roots of Jewish aspects of the Islamic Calendar it is necessary to trace the history of the Jewish Calendar from Temple times (1st Century CE) until the time of Caliph 'Umar 639 CE.