Because it is a solar and lunar calendar (the year follows the seasons, and the months keep track of the phases of the moon), the Jewish calendar has an extra leap month 7 times every 19 years. This leap month occurs in spring the month preceding Passover, so it is appropriate to count the Jewish months starting with Nisan, the month of Passover, as follows:
Since the Jewish year begins in the fall, this numeration of months will not always follow an increasing order; that is, later dates in the same year may have a smaller number for the month: