By Rav Mordechai Gifter, zt"l Translated by Rabbi Israel Schneider, shlit"a
In explanation of the dictum (Megillah 7b): It is incumbent upon a person to cheer himself [with wine] on Purim until he cannot tell the difference between "Accursed be Haman" and "Blessed be Mordechai", some authorities say that these two phrases have the same numerical value (502). The intent of this equivalence Is that one may recognize the Divine not only from the blessedness of Mordechai, and the great miracle which that symbolizes, but also from the cursedness of Haman, i.e., his terrifyingly harsh decree, for that too is a manifestation of the Divine.
I heard this concept from my master and teacher, the Rosh HaYeshiva Rabbi Avrohom Yitzchok Bloch z"l, who explained a verse in Eichah (1:15): "He proclaimed a set time [mo'ed] against me to crush my young men," in the following manner: The word mo'ed, lit., set time, is the word used to refer to each of the various festivals throughout the Jewish year. The festivals are denoted thus, because the word mo'ed is cognate to the word va'ad, which means a convention or assembly. On these festivals, so to speak, we convene with the Holy One Blessed is He, against the backdrop of joy and goodness represented by each particular festival. Likewise, on Tisha B'Av, the day of harsh judgment, we also convene with our Father in Heaven, united with Him in tragedy.
But why is this obligation specific to Purim? We must understand that Purim is that day which is uniquely suited for one to acquire this worldview, for the "miracle" of Purim was one in which G-d's face, so to speak, was concealed. Indeed, the Gemara (Chullin 139b) finds an allusion to Esther in the verse (Deuteronomy 31:18): "But I will have surely concealed my face on that day." It is on this day, then, when we discern the presence of G-d, when He seems to be concealed, that we could raise ourselves to sublime and exalted heights to recognize the hand of G-d, both in Mordechai's blessedness and Haman's cursedness.*
I would like to propose, with G-d's help, the following explanation for the idea that one is obligated to cheer himself [with wine] on Purim until he cannot tell the difference between "Accursed be Haman" and "Blessed be Mordechai."
We must understand that at the root of everything there is no "good" or "evil", no forces and counterforces. Rather, everything that exists and that which transpires all lead to the true and ultimate destiny, as per the Divine Will. This is the selfsame concept for that which the Gemara (Pesachim 50a) teaches: Whereas in this world one recited the blessing, "Blessed are You… Who is good and does good," on good tidings, and the blessing, "Blessed are You… the true Judge," upon bad tidings, in the World-to-Come, even upon bad tidings, one will recite the blessing, "Blessed are You… Who is good and does good." As the Tzlach (ibid.) explains, in the next world we will understand that even that which might be perceived as evil, is in reality good, for it serves, in some way, to further G-d's ultimate goal and mission.
Now, we are enjoined to rejoice on all the holidays and festivals which we celebrate. However, that rejoicing is confined within bounds i.e., limited in its nature. This is because all these celebrations were established as a remembrance for the Exodus from Egypt, in which the supernatural overpowered the laws of nature. The point of the festival, then, is to teach us that we must not view the laws of nature as being immutable, but rather we should understand that as part of G-d's design for the world, the supernatural will occur and trump the laws of nature. Since the celebration marks the integration of the miraculous within the well-ordered laws of nature, it stands to reason that the attendant celebration and joy must also fit into certain circumscribed limits. However on Purim, we do not celebrate the blatantly miraculous, but rather the "miracle" which develops naturally, in which G-d's face, so to speak, is concealed. We must elevate ourselves above the laws of nature, and come to the recognition that these laws which seem so prosaic and natural, are in reality governed from On High. Correspondingly, the joy which we experience on Purim must be elevated beyond all limits. Hence, the obligation to cheer himself until he cannot tell etc.
Based on the above, we can understand the statement: All the festivals will be annulled except that of Purim. In the World-to-Come, in which "the earth will be filled with the knowledge of Hashem as water covering the sea bed" (Isaiah 11:9), it would be redundant to celebrate a festival which commemorates the Divine supernatural intervention into the affairs of the world, and how the supernatural and the natural are in reality but one, for people will then have a heightened cognizance of this concept. However, the festival of Purim, in which we focus upon how the "law of nature" is controlled from the Heavens Above, will continue to exist in that era, as well, with even greater intensity and vigor.
*See the conclusion of the essay, "The Pathway of the Redemption of the Miracle of Purim" at www.kollelateresmordechai.org/purim