Summary of our response up to this point: We cited several sources for the law that we only intercalate Adar – including Bavli (Rosh Hashanah 7a and Sanhedrin 12a), Yerushalmi (Sanhedrin 1:2), and Rambam (Hilchot Kiddush Hachodesh 4:1). However, your question is a good one: Why?
Tosafot (Sanhedrin 12a) offers a scriptural reason: to ensure that Adar will remain the twelfth month, as it is referred to in Megillat Esther (3:7).
We noted the reason for any intercalation – the 11-day discrepancy between the lunar and solar years. The lunar year is 354 days, which is the approximate time it takes for 12 new moons to occur. The solar year is 365 days, which is the approximate time it takes for the earth to complete one solar revolution. Thus every several years, an extra month is added to the Jewish lunar year, allowing the lunar and solar years to be in sync again and ensuring that the holidays are celebrated in their correct seasons.
Last week, we noted that the only logical month to intercalate is Adar. If Nissan or Iyar were intercalated, we would be presented with a problem when counting the omer. We count 49 days starting on the second day of Pesach, Nissan 16, until the omer’s conclusion at Shavuot, the 6th day of Sivan. If we would add a month anywhere between the two, Shavuot would no longer occur in Sivan.
While Shavuot is not necessarily required to fall on a fixed day, Exodus 19:1 specifically states that “in the third month [Sivan] from the exodus of the Children of Israel from the land of Egypt, they arrived in the wilderness of Sinai.” Thus, the Torah is emphatic that the giving of the Torah took place in the third month. As such, Shavuot – when we celebrate being given the Torah – must fall in Sivan.
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We find scriptural mentions of a specific month for other holidays as well. Numbers 29:1 states: “U’bachodesh ha’shevi’i b’echad la’chodesh mikra kodesh yihyeh lachem – in the seventh month [Tishrei] on the first day shall be a holy convocation for you” – this refers to Rosh Hashanah. “U’be’asor lachodesh ha’shevi’i ha’zeh mikra kodesh yihyeh lachem – and on the tenth [day] of the seventh month shall be a holy convocation for you” (ibid. 29:7) – this refers to Yom Kippur. And “U’ba’chamisha asar yom lachodesh ha’shevi’i mikra kodesh yihyeh lachem – and on the fifteenth day of the seventh month shall be a holy convocation for you” (ibid. 29:12) – this refers to Sukkot.
If we were to intercalate any of the months preceding these festivals, they would not occur in the seventh month as the Torah mandates.
The Gemara (Rosh Hashanah 7a) intimates very clearly that we only intercalate right before Nissan because the Torah (Deuteronomy 16:1) instructs us: “Shamor et chodesh ha’aviv v’asita pesach la’Shem Elokecha – You shall observe the month of springtime and perform the paschal offering.” In other words, Pesach must occur in springtime, and if we notice as Nissan approaches that it won’t, we intercalate an additional Adar.
Indeed, the Rambam (Hilchot Kiddush HaChodesh 4:1) writes specifically that intercalation is only done right before Nissan. “[A]nd we make that year two Adars, Adar Rishon and Adar Beit.” He adds, “If not for the addition of this [extra] month, Pesach would fall at times in the winter and at times in the summer.”
Rabbeinu Ovadia, in his commentary, explains that beit din can examine the crops in Adar and see if they have already ripened (a clear sign that spring has arrived), thus obviating the need to intercalate that year.