Seven times, in every nineteen years, another month is added to the Hebrew Calendar. It’s called a leap year (Hebrew: שנה מעוברת).
Leap year is a year containing one additional day, or month in Judaism, in order to keep the calendar year synchronized with the seasonal year.
In Judaism, this extra month is called Adar Alef (first Adar) and is added before Adar, which then becomes Adar Bet (second Adar).
In Hebrew, a year with 13 months is referred to as Shanah Me’uberet, literally: a pregnant year. This is done to ensure that Passover is always in the spring as required by the Torah.
Another reason is that the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. These postponement rules reduce the number of different combinations of year lengths and starting days of the week from 28 to 14 and regulate the location of certain religious holidays in relation to the Sabbath.
For example, the first day of the Hebrew year can never be Sunday, Wednesday or Friday. One reason for this rule is that Yom Kippur, the holiest day in the Hebrew calendar, must never be adjacent to the weekly Sabbath. It must never fall on Friday or Sunday, in order not to have two adjacent Sabbath days. However, Yom Kippur can still be on Saturday.