Each of the three monotheistic religions uses a different type of calendar
The Christian calendar is a solar calendar in which a year is the time it takes Earth to orbit the sun — around 365 days
The Muslim calendar is a lunar one in which a year consists of 12 full cycles of the Moon - roughly 354 days
The Jewish calendar is a lunisolar one. In this calendar, a month is defined by the moon, but an extra month is added periodically to stay close to the solar year. How did the Jewish calendar become lunisolar?
Last Saturday we commemorated the beginning of the month of Nisan, the Exodus which happened on the 15th day of Nisan and is celebrated by the holiday of Passover.
In Deuteronomy 16:1 we read: “Observe the month of Aviv and celebrate the Passover of the Lord your God, because in the month of Aviv he brought you out of Egypt by night”. The Hebrew meaning of “the month of Aviv” is “the month of Spring”, which means that God said that the month of Nisan, in which the holiday of Passover is celebrated, should always happen in Spring. Since this cannot be obtained by a lunar calendar, the Sages concluded that the Jewish calendar should be a lunisolar calendar in which every 3 or 4 years an extra month is added (to a year which becomes a 13 month year), to assure that the month of Nisan will always occur in Spring.
In Exodus 12:2 we read about the month of Nisan: “This month is to be for you the first month, the first month of your year”. Despite the fact that the Jewish calendar year does not start on the month of Nisan, but rather on the month of Tishrei (the seventh month), the counting of the months starts with Nisan. This can be explained by the fact that in this month of Nisan, upon their redemption from Egypt, the Children of Israel became the Israeli Nation. This was our first step as a nation and God ordered us to preserve it by starting the count with this month.