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.

That this never happened proves that intercalation was practiced in biblical times, too. There is no evidence, however, that it was worked out arithmetically in advance, as was done by the Babylonians. In all likelihood, biblical leap years were determined ad hoc; if, for instance, Passover came in a year when spring was just beginning, a month was added, presumably by agreement between the secular authorities and the Temple priesthood, in order to put the holiday back on course. (Rabbinic tradition has it that sometimes it was not a second Adar, as it is now, but a second Elul, the last month before the start of the new year.) Although accumulated experience would have taught more or less how often and when there would be a need for this, there appears to have been no systematic method of predicting it.

Such a method was first developed by the rabbis of the talmudic period and came to be known as sod ha-ibbur, a phrase that should seemingly be translated as “the secret of impregnation”—or, discarding the biological metaphor, “the secret of intercalation,” sod being the Hebrew word for “secret.” This would not have been an inappropriate term for a system of reckoning that was for a long time known only to a few, and it is what the phrase eventually came to mean. Yet it is not what it originally did mean, and thereby hangs a linguistic and historical tale.

This begins with the fact that the word sod in the Bible has a secondary meaning, closely related to its primary one of “secret,” namely, “council” or “assembly.” Thus, when Jacob says of his sons Shimon and Levi, b’sodam al tavo nafshi, most Bible translations render this along the lines of, “Let not my person be included in their council.” (The connection between the two meanings is obvious, a council being composed of persons who confide to each other what they do not share with others.) Indeed, the Talmud tells us, a rabbinic sod ha-ibbur or “council of intercalation” existed in Palestine in the first centuries of the Common Era. We read in the tractate of K’tubot, for instance, that the 3rd-century sage Rabbi Elazar ben Pedat, who was born and raised in Babylonia and settled in Palestine as a young man, was “given a seat” on the sod ha-ibbur after receiving his rabbinic ordination.

Exactly which rabbis belonged to this council, how they were chosen, and what their deliberations consisted of is unknown. We do know that in the year 359 CE Hillel ben Yehuda ha-Nasi, head of the Sanhedrin, the supreme rabbinic body that was then located in the city of Tiberias, decreed that leap years would henceforth be determined by a predetermined schedule rather than by the annual decisions of a council. This schedule was based on a recurrent cycle of nineteen years, seven of them leap years. Long known to the Babylonians and Greeks, and named “the Metonic cycle” after the 5th-century BCE astronomer Meton of Athens, nineteen years were the interval between the moon’s exact phases at a given time of the solar year and their recurrence. The seven intercalations assured, during this period, that the Hebrew months remained in their seasonal place.

Yet though the rabbinic adoption of an intercalated Metonic cycle rendered the sod ha-ibbur, the council of intercalation, supernumerary, the Sanhedrin did not publicly reveal the nature of the new system and continued to announce on a yearly basis whether the year ahead would be a leap year or a regular one. The reasons for its policy of secrecy were several, the main one being the desire to strengthen its authority vis-à-vis the rabbis of the rising Jewish community of Babylonia, which was then vying with Palestine for predominance in the Jewish world. If the Sanhedrin alone knew how leap years were determined, the Jews of Babylonia would have to depend on it for their calendar.

Even after the Sanhedrin’s disbandment in 425, the rabbis of Palestine sought to preserve their prerogative of declaring leap years by not divulging how these were arrived at, so that in the centuries that followed, the sod of sod ha-ibbur ceased to refer to a council and assumed the word’s primary meaning of “secret.” When, for example, Aharon ben Meir (859–929), the leading Palestinian rabbi of his age, spoke of “the sod ha-ibbur that is in our [the Palestinian rabbis’] possession,” this was the sense in which he used the word.

By then, it is true, the secret of intercalation was no longer much of a secret and had long been known to the rabbis of Babylonia, too. Yet some of its details remained in dispute, and Ben Meir made this statement in the Jewish year 4681, the Christian 921, in the context of a controversy with Saadya Gaon, the acknowledged head of Babylonian Jewry, about whether the coming year of 4682 was a leap year or not. Saadya, who held that it was, refused to accept Ben Meir’s opinion that it wasn’t, and insisted on adding a second Adar to 4682’s calendar. Although Ben Meir’s response to this has not been preserved, he apparently backed down to avert a schism in the Jewish world, in which his and Saadya’s followers would have celebrated the Jewish holidays at different times. Summing up the outcome of the debate, in which all the aspects of intercalation were for the first time discussed publicly, the Israeli scholar Rahamim Sar-Shalom wrote in a 1988 article:

The secret of intercalation had now become the public teaching of intercalation. Henceforward, the rules determining the Jewish calendar were set in stone. If up to this point, they belonged to a body of esoteric knowledge that only a chosen few were privy to, from now on they were common property in which not even the slightest change could be made. The Jewish calendar had been given its final form.

This form dictates that the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth years of the Metonic cycle shall be shanim m’ubarot. Since 5784 is the eighth year of a cycle that began in 5777, the Christian 2017, it’s a pregnant one. The child will be born on March 10, the first day of Adar II, eleven days from the 29th of February.