This year, an additional month of holiday whitespace separates the usual four weeks between Tu Bishevat and Purim.
We are in a Jewish leap year, with 13 instead of 12 lunar months, one of seven in every 19-year cycle. (The technical term is “embolismic” or “intercalated” year, but the traditional Hebrew, shana me’uberet — a “pregnant” year — may be the most user-friendly.)
The need for an occasional 13th month arises from a major discrepancy between solar and lunar cycles. On average, there are roughly 29 and a half days between New Moons. But 12 lunar months add up to only 354 days, 11 short of the (again, roughly) 365 and a quarter days of the solar year. Without periodic adjustment, the lunar months would march relentlessly backwards through the seasons, making it difficult for ancient Near Eastern farmers to prepare for critical events on the (solar) agricultural calendar like when to sow grain.
Furthermore, the Torah dates the holidays — except for Shavuot — by the day of the month. On an uncorrected lunar calendar, the festivals might fall in any season. Backsliding months would especially interfere with the observance of Passover, whose first day must always fall in the spring.
The solution is to insert a second Adar into the calendar — “intercalate” — to compensate for the lagging lunar months. In rabbinic times, before the wide adoption of a fixed calendar, intercalation was determined by a Jewish court each year, as needed, based on multiple factors including the (predictable) first day of spring, the ripening of the barley crop and fruit trees, and the condition of the roads leading to Jerusalem, on which Passover pilgrims depended. In theory, the court could decide to add an Adar II as late as the final day of Adar I, only fifteen days before the expected date of Passover (today, depending on your shopping habits, that decision might either be a blessing or a nightmare).
It’s virtually impossible to maintain such a high level of calendrical uncertainty, and we have long since moved to a fixed calendar, including a perpetual intercalation cycle. According to a Geonic tradition, in 359 CE the patriarch Hillel II revealed and mandated those rules, once and for all, for Jews around the world (the reality is more complicated, as controversies continued well into the tenth century).
Hillel’s calendar was based on the Metonic cycle, already in use for centuries in neighboring civilizations. In ancient Babylonia, for example, Nisanu, Ayaru, Simanu, Du’uzu, Abu, Ululu, Tishritum, Arakhsamna, Kislimu, Tebetu, Shabatu, and Adaru made up the regular 12-month year, with a second Adaru intercalated in six of every 19 years (in the 17th year of the cycle, a second Ululu was added instead of Adaru). The Metonic cycle is named after the mathematician Meton of Athens (fifth century BCE), who tried but failed to convince the Athenians to use it. This system was later officially adopted by the Seleucid Greek empire.
That the Hebrew months sound conspicuously like their Babylonian counterparts is no coincidence. The Jerusalem Talmud already recognized that “the names of the months came up with them from Babylonia,” since they appear only in biblical books written during or after the Babylonian exile.
But beyond month names, the entire underlying framework of our lunisolar calendar appears to be — from a narrow perspective — of “foreign” origin. There is no reference in the Bible to a second Adar and while the calendar’s rules are halakhically motivated, it is implausible that the Metonic cycle was an independent rabbinic innovation.
Still, the story of the 13th month is a uniquely Jewish one.
Like the lunisolar calendar itself, its months in a perpetual state of falling behind and running ahead of the seasons, Jewish society has always been engaged in the push and pull of resisting and embracing its surrounding cultures. Over the centuries, the calendar — like many aspects of Jewish life — has been the subject of periodic refinement, and occasional controversy, as it adapted to new scientific, economic, and social realities, while remaining loyal to religious tradition.
“With the arrival of Adar, we increase our rejoicing,” goes the talmudic adage. We celebrate Purim in Adar II, because it is considered the true Adar, while Adar I is the bonus month. Although 14 Adar I appears on some calendars as Purim Katan (Minor Purim), the full observance and joy of Purim belongs to Adar II.
But there is good reason to celebrate early this year, and every leap year, in honor of the unsung Adar I. We can take great pride in our intercalated month, designed to synchronize the moon with the sun, and Jerusalem with Babylon, if only temporarily and imperfectly.