The Ramchal writes (Derech Hashem Chap. 7), in explaining the Moadim, that the Jewish calendar is cyclical, not linear. Therefore, the events that took place during each of the Yomim Tovim made an indelible impression on the calendar cycle, and when the cycle comes back to that point of the year the Divine Light of that time resurges as well.
The holidays are not merely historical commemorations of ancient events, rather they are resurgences of the Godly inspiration that was created during the original event.
Therefore Pesach is an opportunity to tap into the bounty of personal emancipation and freedom of the Exodus, Shavous is a time to have Divine assistance in Torah learning and so on.
The Ramchal continues: the cyclical inspiration of Purim is in the salvation of the Jews during the Babylonian exile, and in that they reaccepted the Torah in a permanent way.
What does he mean when he writes that we reaccepted the Torah?
The Gemara in Shabbos (88a) explains that when we stood at Har Sinai Hashem held the mountain over our heads like a barrel and delivered an ultimatum. Either accept the Torah or this will be your burial place! The Gemara notes that from here we have a really good excuse when challenged on our lack of fulfillment of the Torah’s laws – we were forced into it!
However, the Gemara goes on, that in the days of Achashverosh we reaccepted the Torah willingly (and are therefore bound, with no out).
So presumably, the Ramchal is referencing this Gemara, and telling us that each year on Purim we once again have a unique ability to reaccept the Torah in a willing manner as we did on that first Purim.
There are many other manifestations of this concept in the customs and laws of the day of Purim.
The Rema writes that the custom is to wear Shabbos clothes on Purim (and as an aside, the Poskim stress that this is especially important during the Megilla). The Chida explains that the reason for wearing extra nice clothing is for the Simchas HaTorah that we have on Purim.
The Behag also refers to Purim as being comparable to the day the Torah was given, and his source is the above Gemara.
The Shelah refers to Purim as the ‘conclusion’ of the acceptance of the Torah, and the joy of Purim is the joy of Simchas Hatorah.
The Rema codifies: (695:2) One should engage in Torah study before beginning the Seudah as it says “The Jews had Orah Vesimcha” and Orah (light) refers to Torah. The Levush notes it should be specifically before the Seudah, first Orah, then Simcha.
Wishing you all a wonderful, inspiring and Torah filled Purim!