(Exodus 12:2) "the beginning of months": I might think, for the minimum of months, two (i.e., the most distinctive of months, Sivan and Tishrei). It is, therefore, written (Ibid.) "of (all) the months of the year." (Exodus 12:2) "It is the first": What is the intent of this? It is written (Devarim 16:1) "Observe the month of Aviv (spring) and offer the Pesach (Passover) to the L–rd your G–d, etc." Observe Pesach for Aviv, and Aviv for Pesach, that Aviv fall out in its proper time. How so? Intercalate Adar, so that Aviv fall out in its proper time. If they intercalated Adar and Aviv did not fall out in its proper time, I might think that they should intercalate the New Moon of Nissan. R. Yishmael says: If you say this, you make Nissan twice; but the Torah has said: "It is the first (i.e., one) to you" (and not two).