Jewish Leap Years
Everyone thinks Judaism is based on a lunar calendar. It is true that each Hebrew month has 29 or 30 days based on the cycles of the moon around the sun, which lasts 29.5 days.
But because there is much importance placed on Jewish holidays landing in the right season, the calendar also incorporates the solar year by adding a month every three years to synchronize with the solar calendar.
This year, 5768, is one of those years, a shana meuberet, or pregnant year. And the month that we add is Adar, known officially as Adar Aleph.
There are two explanations given for why we add an Adar. The first is that in ancient times Adar was the 12th, rather than the sixth, month. As such, it made sense to add the extra month at the end of the year.
The other reason is that Passover takes place in Nisan, the month following Adar, and is meant to be in the spring. Putting in an extra Adar pushes the holiday that much further away from the winter.
The official formula for calculating the Hebrew calendar was introduced by Hillel in 359 C.E. With this formula, once every 19 years the solar year and the Hebrew year align, and a new cycle begins.