The intricacies of the Hebrew calendar leap year

No, it isn’t nearly as rare as “Thanksgivukkah,” the once-in-75,000-years overlap of the first day of Hanukkah and Thanksgiving Day that took the Jewish world by storm in 2013. But this year’s 13-month Hebrew calendar isn’t an annual occurrence either.

As February turned to March on the Gregorian calendar this year, the Hebrew month of Adar Aleph transitioned into Adar Bet. The incidence of a second Adar comes up seven times every 19 years on the Hebrew calendar.

Traditional lore attributes the standardization of the Hebrew calendar–in which the months represent the course of the moon but must be aligned with the seasons of the year–to Hillel II, the leader of the Jewish Sanhedrin in the 4th century, but experts believe the evolution of the calendar was much more gradual.

“The Bible contains some basic references to solar and lunar elements, but it does not lay out clear rules…. By the rabbinic period the calendar looked similar to the one we use today, although there were sectarian groups who did not accept it,” Elisheva Carlebach, professor of Jewish History, Culture and Society at Columbia University in New York City, told JNS.org.

Sasha Stern, head of the Department of Hebrew and Jewish Studies at University College London and author of “Time and Process in Ancient Judaism,” told JNS.org that “a lot of people use the word ‘lunisolar’ to indicate that the calendar is regulated by the moon (which defines the beginning of the month) as well as by the sun (which demands the addition of 13th lunar month every two or three years).”

A core aspect of the original establishment of a Hebrew calendar was the need to determine the timing of biblical and religious holidays, such as Passover, Yom Kippur, and Rosh Hashanah. Later, non-biblical holidays such as Purim and Hannukah, and even Israeli Independence Day, were added to the calendar.

“In the Jewish calendar, the addition of a 13th month is required for keeping up with the seasons (e.g. spring for Passover), not with the sun,” said Sasha Stern, head of the Department of Hebrew and Jewish Studies at University College London. In meteorology, the schedule of the seasons does not correspond with the movement of the sun.

The lunar year is 12 lunar months of an average of 29 and a half days each, with a total of approximately 354 days in a year, Stern explained. “This falls short of the seasons by about 11 days,” and thus “an extra month needs to be added every two or three years in order to make up for this and keep up with the seasons,” he said.

The ancient Israelite calendar was most likely lunar, with 12 months in the year, each of which begins with a new moon. Stern said all lunar calendars in the world “have always added a 13th (leap) month,” with the exception of the Islamic calendar.

 A fixed calculation of the Hebrew calendar was finalized in the 10th century. Some diversity in how the calendar was applied persisted well into the medieval period, but the fixed Hebrew calendar became largely universal over time.

In her book “Palaces of Time: Jewish Calendar and Culture in Early Modern Europe,” Carlebach describes that following the 15th century in Europe, Jews began to treat calendars not only as conceptual measurements of time, but as material things. Manuscripts of the Hebrew calendar began to circulate in various forms. The printing revolution allowed for the reprinting of the calendar not only by Jews but also by Christians.

The Jews took note of Christian holy days in their calendars, both to avoid potential acts of persecution, which tended to occur more often on holy days, but also to trade with Christians, whose market fairs often took place on holy days.

Today there are calendars on our phones and on the Internet. The medium is less important than its durability for the time it is needed and its portability,” Carlebach said.

While the age-old intricacies of the Hebrew calendar aren’t novel, the calendar is gaining newfound relevance in Israel today. A recent Knesset bill stipulates that official identification issued to Jewish citizens by a public authority should use Hebrew calendar dates, instead of the Gregorian dates commonly used worldwide. “This bill, which would increase the use of the Hebrew date, is another step in strengthening the Jewish democratic character of the state of Israel,” Stern said.

ALINA DAIN SHARON (adsharon@jns.org) is the managing editor at JNS. This article was excerpted and reprinted with permission.