The rabbinic mandate, MiSheNichnas Adar Marbim BeSimchah, is, on the one hand, deeply familiar to all of us, and yet, at the same time, halachically speaking, puzzling. Indeed, when one examines the source of the dictum itself, a statement of Rav located towards the end of Masechet Ta’anit, “KeSheim SheMiSheNichnas Av Mema’atin BeSimchah, Kach MiSheNichnas Adar Marbin BeSimchah,” “just as we become less happy when Av arrives, so too we become more happy when Adar arrives,” the difficulty is compounded. After all, the former clause, that the arrival of Av occasions a deliberate and systematic reduction of joy, is readily understandable: the city walls of Yerushalayim had already been breached, and the destruction of the Beit HaMikdash was, at that point, a tragic inevitability.
Yet, the latter clause, mandating an increase in celebration with the arrival of Adar, prima facie, appears without the same kind of historical justification. On the contrary, the Jews of ancient Persia were in no position whatsoever to celebrate when the fateful month of Adar arrived. It was only their victory on the thirteenth of the month which enabled the celebration that would subsequently ensue. Surely, we would have imagined, the celebration ought to have been limited to the days of Purim themselves, with the first thirteen days of the month, if anything, defined as times of national distress and anxiety.
Second, even if we were to disregard the specific events of Purim itself, we do not, in the general sense, find any sort of parallel injunction concerning the rabbinic institution of Chanukah, in the spirit of, “when Kislev arrives, we begin to increase our joy,” raising further questions concerning the source of this particular Halachah. Likewise, at the Torah level, we do not find any such concept regarding Shavu’ot.
Finally, in his brief comment on the Gemara, Rashi further complicates matters by surprisingly incorporating Pesach into the discussion: “Yemey Nisim Hayu LeYisrael: Purim U’Pesach,” “Purim and Pesach were days of miracles for Yisrael.” It would seem, based on the simple reading of Rashi, that whichever expressions of joy that are triggered by the mandate of MiSheNichnas Adar Marbim BeSimchah ought to continue through the end of the month, and into Nissan as well. And yet, Rashi does not seem to address the fundamental question at stake, namely, the reason that these expressions of joy should commence with the arrival of Rosh Chodesh Adar.
In light of these difficulties, it certainly bears mentioning that whilst Rambam codifies the first clause of Rav’s statement, MiSheNichnas Av Mema’atin BeSimchah, he pointedly omits any mention of MiSheNichnas Adar Marbim BeSimchah. The same can be said for Tur and Shulchan Aruch as well.
It seems to me that some perspective on this question may be gained by examining the precise nature of Haman’s lot. While, admittedly, the text is somewhat ambiguous, it seems likely that Haman cast his lot only with respect to the month in which he would seek the destruction of the Jewish people, not the day. The Pasuk states that Haman cast lots “MiYom LeYom UMeiChodesh LeChodesh Sheneim Asar, Hu Chodesh Adar,” “[concerning] every day and month [until the lot fell on] the twelfth month, the month of Adar.” Remarkably, the date of the intended destruction is not found in the text at all. The succeeding verses in the chapter reveal that Haman immediately approaches the king, the decree is issued, and the day merely happens to be the thirteenth of the month of Nissan.
As such, one can reasonably argue that Haman selected only the month of Adar without specifying the date. The precise date, namely the thirteenth, emerged idiosyncratically, purely as a result of the fact that it happened to be the thirteenth day of the month of Nissan when the lot was cast. This reading may be confirmed by a striking passage in the Gemara which relates that Haman was elated when the lot fell on the month in which Moshe died. While the Gemara goes on to wryly note that Haman was oblivious to the fact that Moshe was also born in that month, the entire premise of the Gemara is sensible only if we understand that Haman was singularly focused on the month of destruction, as opposed to the date. Had Haman been focused on the date as well, his reaction should not have been elation but frustration, as he had missed out on the date of Moshe’s death, Adar 7th, by a mere six days, an experience akin to having four of five correct lottery numbers.
If it is indeed the case that Haman selected the month of Adar for the destruction of the Jewish people, while the date was merely a byproduct of the date upon which he happened to draw the lot, the expression towards the very end of the Megillah is far more understandable, “HaChodesh Asher Nehpach Lahem MiYagon LeSimcha UMeiEivel LeYom Tov,” “the month that was transformed for [the Jews] from grief to happiness and from mourning to festivity,” with emphasis on the month of Adar, not the day. On the basis of this Pasuk, the Talmud Yerushalmi derives a shocking but profoundly illuminating Halachah: in theory, one may fulfill his obligation to read the Megillah at any point during the month of Adar. While the Talmud Bavli does not go quite as far as the Yerushalmi, the very institution of “Kefarim Makdimin LeYom HaKenisah,” permitting villagers to read the Megillah as early as the 11th or 12th of the month, equally points in the direction of a holiday localized less to two particular calendar dates than to an entire month: HaChodesh Asher Nehpach Lashem MiYagon LeSimcha.
The argument that the basis for Rav’s extension of the Mishnaic statement, MiSheNichnas Av Mema’atin BeSimchah, to MiSheNichnas Adar Marbim BeSimchah is rooted in Haman’s own lot may be strengthened by the following asymmetry between two cases. While there is a host of Halachic expressions of the reduction of joy commencing with Rosh Chodesh Av, ranging from commercial activities, certain forms of planting and construction, and holding weddings, there is only one Halachic expression concerning increasing joy in the month of Adar: if a Jew has a legal dispute with a Nochri, he should feel most confident adjudicating the matter in Adar. Even this particular expression of MiSheNichnas Adar Marbim BeSimchah seems patterned, albeit at far less of an existential plane, on the confrontation between Haman and the Jewish people.
And yet, even if the transformation of the month of Adar can be traced towards Haman’s lot itself, one might still argue that Chazal needed a precedent for an entire month to be transformed beyond the immediate days of celebration themselves. It is in this connection that Rashi’s aforementioned insertion of Pesach in his explanation of the concept of MiSheNichnas Adar Marbim BeSimchah might be particularly instructive. Indeed, if there is a model for an entire month that is transformed beyond the immediate days of celebration contained within it, Nissan is certainly the paradigm. The restrictions on eulogies and recitation of Tachanun during the entirety of the month of Nissan, not limited to the days of Pesach themselves, may be conceptualized as a halakhic precedent for the transformation of an entire month, a precedent upon which Rav’s mandate, MiSheNichnas Adar Marbim BeSimchah, may well rest.
Furthermore, Rabbi Shimon ben Gamli’el’s insistence, contra Rabbi Eliezer beRabbi Yosi, upon reading the Megillah during a leap year in the Adar which immediately precedes Nissan certainly underscores the fundamental connection between these months. It is certainly reasonable to interpret Rabbi Shimon ben Gamaliel’s stated reason, “Mesameich Ge’ulah LeGe’ulah Adif,” “it is preferable to juxtapose the redemption [of Purim] to the redemption [of Pesach],” on a more superficial plane, that both of these months contain redemptive moments for the Jewish people. However, I prefer to interpret this Halachah as a reflection of a more profound bond between Adar and Nissan, namely that the two months that have been transformed above and beyond the specific days of celebration contained therein. In this sense, the very words employed by Rabbi Shimon ben Gamli’el, “Mesameich Ge’ulah LeGe’ulah,” can be interpreted in a far more precise way, not merely as connoting a general proximity between Purim and Pesach, but, quite literally affixing one month of redemption directly to the other.
While the rabbinic nature of both of these institutions, the prohibition against eulogies throughout Nissan and the definition of the entire month of Adar as one of happiness, precludes a direct application of the concept of “Kol DeTikkun Rabanan KeEin DeOraita Tikkun,” that Rabbinic laws are patterned after Torah laws, the conception that Adar, as a month of celebration, was patterned after Nissan is certainly an analogue of this principle. The fact that the critical events of the Megillah--the three-day fast, Esther’s approach to Achashveirosh, and the exposure of Haman--occurred on the days of Pesach themselves renders this connection that much more compelling.
Whatever its origins, Rav’s halakha of MiSheNichnas Adar Marbim BeSimchah lends itself to one final interpretation. As the celebrated passage in Masechet Shabbat details, during the generation of Achashveirosh, “Kiyemu Aleihem Mah SheKiblu Kevar,” the Jewish people reaffirmed their commitment to the eternality of Torah. A nation on the verge of total assimilation, whom the Talmud pointedly notes were fully represented and engaged in the debased orgy of Achashverosh, rediscovered its spiritual footing. A people rightly accused of being “Mefuzzar UMeforad Bein HaAmmim,” a fractious and discordant group, rediscovered its fundamental unity in three days of spiritual awakening, a unity which harkened back to the singularity of purpose originally manifested at Sinai, when they were described “KeIsh Echad BeLeiv Echad,” “like one man with one heart.” Inasmuch as the Jews of Shushan reconnected to the Torah, it may not be entirely out of place to suggest that the happiness of Adar relates to the ultimate source of joy, Torah, as is written in Tehilim, “Pikkudei Hashem Yesharim, Mesamchei Leiv,” “the precepts of Hashem are just, rejoicing the heart.”
 Talmud Bavli Ta’anit 29a
 Cf. Eliyah Rabbah Orach Chaim 685, who indeed argues that “Nisan is like Adar” in terms of it being a favorable time to pursue litigation against a Nochri in court.
 Note, for example the Sefat Emet (Ta’anit 29a), who rejects the link between Adar and Nissan, and interprets Rav’s statement to be an reference to the Beit HaMikdash: just as the mourning of Av centers around the destruction of Mikdash, the happiness of Adar stems from the collection of Shekalim for the upkeep of the Mikdash.
 Esther 3:7
 One might read the term MiYom LeYom as suggesting a lot cast for the date in addition to the month. However, one wonders, then, why the date is omitted from the end of the Pasuk, while the month is reported. Second, it is quite a coincidence that of thirty possible dates in the month of Adar, the lot happened to fall precisely on the thirteenth, the very day it happened to be in the month of Nissan when the lots were cast.
 Talmud Bavli Megillah 13b
 Esther 9:22
 Talmud Yerushalmi Megillah Perek 1
 Talmud Bavli Megillah 2a
 Shulchan Aruch Orach Chayim Hilchot Tish'ah BeAv UShe'ar Ta'aniyot 551:1-2
 Talmud Bavli Ta'anit 29b, Magen Avraham Orach Chayim 686:5
 Shulchan Aruch Orach Chayim 429
 In fairness, the Chanukat HaMizbei’ach, covering the first twelve days of Nissan, is a factor in the transformation of the month of Nissan in its entirety--in combination with the days of Pesach, the majority of Nissan is festive, and one may employ the concept of Rubo KeKulo, following the majority, to transform the rest of Nissan. Yet, this makes the transformation of Adar, in which there are only two days of celebration, that much more remarkable.
 Talmud Bavli Megillah 6b
 Talmud Bavli Shabbat 88a
 Talmud Bavli Megillah 12a
 Esther 3:8
 Rashi Shemot 19:2 s.v VaYichan Sham Yisrael
 Tehilim 19:9