There is an argument for each side. There is a concept in Jewish law referred to as, ain ma’avirin al hamitzvos (literally, “don’t pass over the mitzvos”). When it comes to doing a mitzvah, we like to do it as soon as possible. Therefore, since Adar Rishon can potentially be the month in which Purim could fall, why push off Purim until Adar Sheni? After all, in non-leap years, Purim falls in the Adar that follows the month of Shevat, so why not celebrate Purim in Adar Rishon which follows the month of Shevat?
To counter this argument, the Talmud introduces a new concept: smichas geulah l’geulah—juxtaposition of one redemption and another. True, Adar normally follows Shevat; however, in a non-leap year, Purim also falls thirty days in advance of the fifteenth day of Nissan, the day on which we celebrate Pesach, as the Talmud points out:
From Purim to Pesach is thirty days ... (Sanhedrin 12b)In a leap year, however, if Purim is observed in Adar Rishon (which gains a day), then the time separation between Purim and Pesach goes from thirty days to sixty days (17 days of Adar 1, 29 days of Adar 2, 14 days of Nissan).
To the Talmud, apparently, this is unacceptable. For this reason, in a leap year, we overlook the all-important reason of ain ma’avirin al hamitzvos, and celebrate Purim in Adar Sheni. However, now the question is, since when does a “theme” override a halachic principle?
The same tractate, Megillah (which deals with the holiday of Purim), also makes another statement that, on the surface, seems quite innocuous. The Talmud states that thirty days in advance of every Jewish holiday, one should make a point of beginning to learn the halachos of the upcoming holiday, to become sufficiently familiar with them. [Megillah 4a. This is learned from the fact that in the desert, the Jews who were unable to celebrate Pesach on the fifteenth day of Nissan were told that in thirty days, on the fifteenth of Iyar, they were to celebrate what would become known as Pesach Sheni (Pesachim 6a).] However, if one make a simple calculation, he will see that thirty days in advance of Pesach is Purim...
From Purim to Pesach is thirty days. From Purim, one should start learning the laws of Pesach. (Sanhedrin 12b)
... But who learns Hilchos Pesach on Purim?
The idea of learning the laws of one holiday while celebrating a different holiday is, in itself, unusual. However, given the nature of Purim itself, and the way it is celebrated (running around delivering Mishloach Manos and becoming inebriated), it is difficult to comprehend how one would be expected to fulfill this law. Is thirty days in advance of Pesach to be taken literally in this case?
The answer is yes! and even though this could be fulfilled by merely taking fifteen minutes from one’s busy day to learn a halachah or two in advance of Pesach, perhaps there is another meaning to this law when seen in light of the concept of geulah l’geulah. Perhaps the rabbis of the Talmud are hinting to us that, anyone who truly understands the meaning of Purim and what it is meant to accomplish; anyone who really understands what the Jew is meant to achieve through Purim, and celebrates Purim accordingly will, in fact, have begun Hilchos Pesach.
For, what is Purim but an important spiritual threshhold to be crossed on the way to the freedom of Pesach? Why else would the rabbis of the Talmud insist that it is more important to juxtapose the celebration of the redemption of Purim with the celebration of the redemption of Pesach, though it means delaying an important mitzvah for thirty whole days? However, the starting point to making sense of this is knowing that, when it comes Purim, nothing is as it seems to be.