We learned in the Mishna (2a) that the month of Nisan is the first month of the year with regard to counting the months. Our Gemara searches for a source for this law, beginning with the command in the Torah that declares the month of the exodus from Egypt to be the first month of the year (see Shemot 12:2). The difficulty in establishing this as a definitive source stems from the fact that the names of the months that are currently in use in the Hebrew calendar are never mentioned in the Torah. Thus the Gemara turns to Nakh (the Nevi’im – Prophets, and Ketuvim – Sacred Writings) to identify the months of the calendar by name and numbering:
- Ravina points out that in Sefer Zekhariyah (1:7), for example, Shevat is referred to as the eleventh month.
- Rabba bar Ulla notes that in Megillat Esther (2:16) Tevet is referred to as the tenth month.
- Rav Kahana shows that in Sefer Zekhariyah (7:1) Kislev is mentioned as the ninth month.
- Rav Aha bar Ya’akov points to another passage in Megillat Esther (8:9) that refers to Sivan as the third month.
Finally, Rav Ashi quotes a pasuk from Megillat Esther (3:7), which first calls Adar the twelfth month and then goes on to clearly call Nisan the first month.
The Gemara asks: And all of the others, what is the reason that they did not say that it is derived from here, the last verse mentioned, which is explicitly referring to Nisan as the first month? The Gemara answers: It is because one could perhaps have said: What is meant here by first? It means the first in relation to its matter, i.e., the months of the decree, and so it cannot be proven from here that Nisan is the first of the months of the year.
Even though the last passage brought up by the amora’im is the clearest one of all, the Gemara explains that it does not constitute the best proof, since it may simply be saying that Nisan was the beginning – the first month – of Haman’s plot against the Jews. The Maharsha points out that according to this logic, none of the months mentioned in Megillat Esther can act as sources, since all of them may be counting from the beginning of the plot against the Jews. Although he leaves the question standing, this is a topic already discussed by rishonim. The Ritva explains that it may be common to announce a given date as the beginning of a particular happening, but it would be unusual to count from that date later on in the process. The Rosh argues that once it was established when the process began, it would be without purpose to repeat it over and over again.
Tosafot quote the Yerushalmi, which points out that the underlying assumption in this Gemara is that we rely on the tradition that the people had regarding the order of the months as we know them today. Without that piece of information, the entire proof-text of pesukim makes no sense.