Adar II

Adar II is the additional, thirteenth month inserted into the Hebrew year in a leap year, falling toward the end of winter and counting thirty days. The Hebrew calendar is lunisolar: twelve lunar months fall about eleven days short of the solar year, so in seven years out of every nineteen a whole extra month is added to keep the festivals from drifting through the seasons, and it is Adar that is doubled. In a leap year the first Adar becomes an ordinary month and Purim is kept in Adar II, on its fourteenth, so that the festival stays in the month adjoining Nisan and Passover. This intercalation is what holds Passover in the spring, as the Torah requires. These are the features of the Jewish calendar tradition, in which Adar II is the mechanism that reconciles the lunar months with the solar year.

Adar II exists only in leap years, seven times in every nineteen-year Metonic cycle used to fix the Hebrew calendar, and in those years it, not the preceding Adar I, carries Purim on the fourteenth and the fast of Esther on the thirteenth, along with all the other observances ordinarily associated with the single month of Adar in a regular year. The doubling of Adar rather than of some other month reflects a specific halakhic concern: inserting the extra month here keeps Nisan, and with it Passover, anchored to spring, since Passover's biblical date is explicitly tied to the barley harvest and must not drift into winter, the very problem intercalation exists to prevent across the whole lunisolar Hebrew calendar.

The name is a Hebrew formation on Adar, itself borrowed from the Akkadian Addaru, rather than a separate Babylonian month name. The month runs a fixed 29 days in every year.

The month names are Babylonian. Twelve of them are the Akkadian month names taken into Hebrew during and after the exile, and the biblical books of the return use them freely while the earlier books generally number the months or use a small set of older Canaanite names. The atlas records the borrowing rather than presenting the names as native, because it is one of the clearest visible marks the exile left on Jewish practice.

The Jewish calendar as now kept is calculated rather than observed. Month lengths are fixed by rule except for Cheshvan and Kislev, which absorb the adjustment and may run 29 or 30 days, and a thirteenth month is added in seven years out of nineteen on a fixed cycle so that Passover stays in the spring. A further set of rules, the postponements, shifts Rosh Hashanah off certain weekdays, which is why the year has six possible lengths rather than two.

Adar II is the added month of a leap year, present in seven years out of every nineteen. Which of the two Adars is the added one is a real question in Jewish law, since documents, anniversaries and the observance of Purim all have to be assigned, and the resolution is that Purim is kept in Adar II while the first Adar is treated as the interpolation.