"In spirituality, we advance along three lines.

These three lines are reflected in our world during the period of concealment as the three religions: Islam corresponds to the right line, Christianity - to the left line, and Judaism - to the middle line.

This also has an affect on how their calendars are structured: The Islamic calendar is lunar (based on the moon), the Christian calendar is solar (based on the sun), and the Jewish calendar is luni-solar (based on both). How did people know what kind of calendar to create? They did it because everything in our world comes from the spiritual roots, and people’s acts are determined by the expression of these roots in our world."

Source:

http://www.laitman.com/2008/12/the-spiritual-roots-of-the-three-religions/