How are the months on a Jewish calendar related to a Gregorian calendar?
The Jewish calendar is based on both the moon and the sun. A month can have 29 or 30 days (to start each month with a new moon), and there can be 12 or 13 months to a year. In every 19 years, 12 of the 19 years have 12 months, while seven have 13 months, thus keeping in line with the solar calendar and making every 19 years on a Jewish calendar exactly equal to 19 years on the Gregorian calendar.
