In light of all the above, why does Tosafot (in Sanhedrin 12a) cite a verse in Esther to explain why we only intercalate Adar?
Moreover, I find Tosafot’s proof difficult. The verse he cites is “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” (Esther 3:7). But surely that year was a shanah peshutah – a simple year with just twelve months, which is why Adar is referred to as the 12th month (which it would not if it were a leap year)
Yet, as we will see, this is not a simple matter.
(To be continued)