Why does the Jewish leap year always consists of two Adars? Why specifically Adar?
Answer: Both the Babylonian Talmud (in two places: Rosh Hashanah 7a and Sanhedrin 12a) and the Jerusalem Talmud (Sanhedrin 1:2) state that we only create leap years during Adar. The Rambam (Hilchot Kiddush Hachodesh 4:1) codifies this as the halacha. Seemingly everyone, even a young child, is aware of this fact. However, your question is a good one: Why only Adar?
A baraita (Sanhedrin ibid., which is the source for this halacha) states: “We do not intercalate the year before Rosh Hashanah [in Elul], and if they did intercalate, it is [not valid]. However, in times of duress, they may intercalate immediately after Rosh Hashanah. Even so, the intercalation is done for the month of Adar.”
Not only is the intercalation restricted specifically to the month of Adar, but beit din (in earlier times) was only allowed to meet in Adar regarding the intercalation. The Gemara (Sanhedrin ibid.) makes note of this and the unusual exception that was made when Rabbi Akiba was in prison. The sages joined him there and proclaimed not one but three (successive) intercalations (for three different years). The Gemara explains that after sitting with Rabbi Akiba, they then met and sat again as a beit din in Adar.
Rashi (ad loc. sv “ein ma’avirin”) explains: “Beit din does not sit before Rosh Hashanah to delve into the matter and proclaim that this year be intercalated with two Adars because there will be forgetfulness due to the [long] lapse of time, with the result that people will come to be careless with chametz [on Passover].”
Tosafot (Sanhedrin 12a, sv “Ein me’averin ela Adar”) offer a scriptural reason for intercalating Adar specifically: “We do not intercalate [any of] the other months [save for Adar] because of the following verse (Esther 3:7) ‘[Bachodesh harishon hu chodesh Nissan bishe’nat shteim esreh la’melech Achashveirosh hipil pur, hu hagoral lifnei Haman, miyom l’yom u’me’chodesh] l’chodesh shneim asar hu chodesh Adar – [In the first month, which is the month of Nissan, in the twelfth year of King Ahasuerus, they cast pur, that is the lot, before Haman from day to day, from month to month,] to the twelfth month, which is the month of Adar.’ Now if we are to intercalate one of the other months, then Adar will not be the twelfth month.”
We must note the reason for intercalating altogether: the discrepancy between the lunar and solar years. Simply put, the lunar year is basically 354 days, which is the approximate time it takes for 12 new moons to occur. On the other hand, the solar year is basically 365 days, which is the approximate time it takes for the earth to complete one solar revolution. There remains an approximate discrepancy of 11 days between the two. Thus, every several years, the cumulative missing days are combined to form an additional month of Adar, delaying the next lunar year from starting again and allowing the lunar and solar years to be in sync once more.
It is interesting to note that many other peoples follow a lunar year. The most prominent examples are the Muslims and Chinese. Since Muslims do not have a leap year, their calendar proves to be quite interesting in that their principal month-long fast and feast, Ramadan, occurs in different solar months in different years. Not so the Jewish year, where the lunar year is adjusted with extra months to ensure that the holidays are celebrated in their correct seasons.
(To be continued)