The Jewish "leap year", which occurs seven times in a
19-year cycle, has 13 months instead of the regular year's 12. This is so that the
lunar-based Jewish year should remain aligned with the solar seasons (12 lunar months
make up a total of 354 days -- slightly more than 11 days short of the 365.25 day solar cycle).
The added month is called "Adar I" and is inserted before the month of Adar (termed "Adar II"
in leap years).
See The 19-Year Marriage.