Hebrew calendar is based on the lunar cycle. Torah instructs us how to devise a calendar based on the appearance of the new moon. It also requires the holidays to fall at certain season of the year.

The time it takes for the moon to go around the earth is about 29 ½ days. A lunar year is about 11 days shorter than a solar year. Our sages devised the Hebrew calendar to compensate for these shortages by adding an extra month in some years. Every nineteen-year contains seven leap years to keep the season intact. Passover holiday is to fall in the spring and Rosh Hashanah, Yom Kippur and Sukkot are to be observed in the fall.

There is another requirement that Yom Kippur cannot fall on Friday or Sunday. Therefore some months have a variable length of either 29 or 30 days to adhere to this rule.