How Jewish Leap Years Work and Why They're So Important

Leap years are added to the Jewish calendar on account of the gap between the solar and lunar years. The Earth completes a full orbit around the sun in 365 days, while 12 lunar cycles only amount to approximately 355 days, the Hebrew alphanumerical value of the word shana (year). The Jewish calendar is built on both solar and lunar years: Rosh Chodesh (the new month) is determined by the lunar cycle, but the holiday of Pesach is celebrated in the spring, which is influenced by the solar cycle.

Each year, the gap between the lunar and solar calendars is about 10 days (since each lunar cycle isn't exactly 30 full days), which indicates that a leap year must be added approximately every three years. In Jewish law, the leap year pattern is based on a 19-year cycle that can be remembered with the alphanumerical mnemonic guch adzat, which indicates that within 19 years, leap years occur in the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years.

In the days of Avraham, Shem, the son of Noach, was responsible for determining leap years. For this reason, when the angel told Avraham that Sarah would have a son "at this time next year," he meant 13 months later. Indeed, the following Pesach was thirteen months later since Shem had declared a leap year.

After the Torah was given to the Jewish people, the Sanhedrin (grand Jewish court) assumed responsibility for determining leap years. Every year, the Sanhedrin and the Sages of the generation would gather for a meeting called sod ha'ibur (the secret of intercalation), where they would decide if the year would be a leap year.

Leap years can only be declared in the Land of Israel. However, if there is a Sage outside of Israel who doesn't have a counterpart in Israel, he can declare a leap year abroad. During the days of the Roman persecution in Israel, Rabbi Akiva traveled abroad and declared a leap year from there, since he was the greatest sage of his generation and "had no counterpart in the Land of Israel."

A leap year can only be implemented during the month of Adar because the addition needs to be made at the end of the Jewish calendar year. Nissan is the first month of the year, and intercalation can only happen right before the onset of Nissan. The Sages learn from the verse that only "Adar adjacent to Nissan" can be intercalated.

Chizkiyahu, the king of Yehudah, lived during the time of the prophet Yeshayahu and succeeded his wicked father, Achaz. Chizkiyahu led a religious revolution and helped the entire nation repent. Since they didn't have enough time to purify themselves before the holiday of Pesach, he decided to intercalate the month of Nissan, and add an additional month of Nissan, but the Sages disagreed with this action.

Yeravam, the son of Nevat, was the first king of Israel's Northern Kingdom. He rebelled against Rechavam, the son of Shlomo, the king of Yehudah. He also altered the months in violation of Jewish law and moved the festival of Sukkot forward by a month. Accordingly, the kingdoms of Yehudah and Israel celebrated the holiday in different months until the destruction of the Kingdom of Israel.