Rav Itamar Shwarz, the author of the Bilvavi Mishkan Evneh.
On one hand, Purim is the last of the festivals, and on the other hand, it is a new beginning (as it is with all “ends”, where the end is always a beginning to something else). The Sages state all of the festivals in the future will cease, except for Purim. This is because it is the end of the festivals of the current time period – and it is a beginning of the future.
Therefore, Purim is intrinsically different than all the other festivals. Purim contains both the light of the current festivals, as well as an additional light – the light that is beginning of the future times.
This additional light contained in Purim stands out in all of the events of Purim and in its unique mitzvos. There are many examples of how we can see it – here is a list of a few of them.
1) The system of the “festivals” begin with Pesach, the exodus from Egypt, where we were told, למען תדע,“So that you shall know”; and on Sukkos as well, with the mitzvah of sukkah, the Torah says that it is למען ידעו דורותיכם, “So that the generations will know.” But Purim is not for the purpose of knowing – it is about עד דלא ידע, “ad d’lo yoda” – it is about “not” knowing [its concept is “above” the normal daas\knowledge].
2) Regarding all mitzvos of the Torah, there is a rule, “the Torah is not in heaven” (Bava Metzia 59b). But Purim was ‘agreed upon’ in Heaven (Yerushalmi Berachos 67b).
3) When we stood at Har Sinai, there was yirah (awe), for Hashem gave the Torah so that “they will learn to fear Me for all days”. But on Purim, where we re-accepted the Torah, we did so with ratzon (will), which came from ahavah\love [for Hashem], because of the miracles experienced [as Rashi in Tractate Megillah states]. This was ahavah (love), as opposed to just having yirah (awe).
4) In all other festivals, we are obligated in them due to standing at Har Sinai and receiving the Torah. But on Purim we had a different kind of receiving of the Torah, by re-accepting the Torah. Clearly it was not the same acceptance again; it was a much deeper kind of acceptance. It resembled, “A new Torah shall come forth from Me” [the Torah of the future].
All other festivals are rooted in Moshe, who received the Torah from Hashem at Har Sinai. But Purim applied to walled cities from the times of Yehoshua, so it is rooted in Yehoshua.
5) When it comes to the rest of the mitzvos of the Torah, either we give to the poor or to the Kohen. But when it comes to Purim, we give Mishloach Manos to friends, out of love for everyone.
6) The Torah is a ‘masculine’ term, for it is called “Toras Moshe”, who was a man. But the Torah which we received on Purim was wrought through a woman, Esther, and the “Torah” that we received on Purim is collected in “Megillas Esther”.
7) All the other festivals were open miracles, but Purim was entirely hidden miracles. This is because the purpose of Purim was to reveal the hidden, resembling the statement, “Wine enters, secrets come out.”
8) All other festivals have a specific time of the calendar, whereas Purim can fall out either on the 11th, the 12th, the 13th, the 14th, or the 15th. The mitzvos of Purim can be performed on an earlier date than the 14th, resembling the possibility of the redemption being earlier than its time.
9) In all other festivals, there is only one performance of the mitzvos of the festival (and even when it comes to shaking lulav, there is only one mitzvah per 7 days of Sukkos to shake lulav), but the mitzvos of Purim can be performed over a period of two days, which are the 14th and 15th of Adar. This is because the spiritual light of Purim is a “double” light. The 14th of Purim is equal to the number י”ד in Hebrew, which has the same gematria as דוד, symbolizing the end of the festivals, and the 15th of Purim corresponds to the days of Mashiach, whose kingdom will be completed on the 15th of the month.
10) Just as Shabbos is a resemblance of the World To Come and it contains doubles (see Yalkut Shimeoni Shemos 16:261), so is Purim a beginning of the light of the future, thus it is a “double” day.