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The predecessor to the Islamic calendar was similar to the [Hebrew calendar]? in that it was primarily lunar but was kept synchronized with the [tropical year]? (that is, based on the motion of the sun) by the insertion of an additional month when required. In the 9th year after the Hejira? or flight to Medina, Mohammed forbade the insertion of the additional months.
Of all the months in the Islamic calendar, Ramadan is the most sacred, and all Muslims have to fast during the daytime.