The Secrets of the Jewish Leap Year

The newly arrived 2024 is a leap year, having an extra day. It will be accompanied by a parallel leap year—or, as it is called in Hebrew, a “pregnant year,” shanah m’uberet—in the Jewish calendar, in which 5784 will have an extra month. Although the coinciding of a Christian and a Jewish leap year is not that rare, neither is it that common. It will not happen again until 2052, seven leap years and fourteen “pregnant years” from now.

The term shanah m’uberet dates to the talmudic period and is self-explanatory, the year in question having an additional month implanted in it as a child is implanted in the womb. Yet the custom of intercalation or ibbur ha-shanah, “the impregnation of the year,” as a way of adjusting the 354-day lunar calendar to the 365-day solar calendar goes back well beyond talmudic times. It has Babylonian antecedents and was clearly practiced in some form in the age of the Bible, whose months are lunar but whose year is solar, so that biblical holidays like Passover, Shavuot, and Sukkot are linked to specific seasons.

Without intercalation, this link could not have been maintained; the lunar month of Nisan, for example, the month of the spring holiday of Passover, would have slipped back annually by eleven days against the solar year and soon regressed into winter. From there it would have worked its way backward through autumn and summer until it would have returned—temporarily—to its starting point. Indeed, this is precisely what happens in Islam, whose lunar calendar has no leap years and whose holidays have no seasonal associations.