In a recent interview, Rabbi Hershel Schachter is quoted as saying:
In the days of the second Beis HaMikdash, there were many yamim tovim d’rabanan, all of which were batul after the churban habayis, except for Chanukah and Purim. Of all these yamim tovim, only one had a chiyuv seudah: Purim. The Gaonim had the girsa in the Gemara that the reason Purim is different is because it’s yom kabalas haTorah.
What is the source for this statement regarding the girsa of the Gaonim?
It seems that everybody above is right - HaRav Schachter is working off of the Gemara in Shabbat, Gemara in Pesachim, and the She'eltot - as he cites them all in Be'Ikvei HaTzon page 114 (right column).
The reference is seemingly to Pesachim 68b where common editions state:
מר בריה דרבינא כולה שתא הוה יתיב בתעניתא לבר מעצרתא ופוריא ומעלי יומא דכיפורי עצרת יום שניתנה בו תורה פוריא ימי משתה ושמחה כתיב מעלי יומא דכיפורי דתני חייא בר רב מדפתי ועניתם את נפשותיכם בתשעה לחדש וכי בתשעה מתענין והלא בעשירי מתענין אלא לומר לך כל האוכל ושותה בתשעה בו מעלה עליו הכתוב כאילו מתענה תשיעי ועשירי
Mar son of Ravina would fast all year except for Shavuot, Purim and Erev Yom Kippur. Shavuot: the day the Torah was given. Purim: it says "days of feasting and happiness". Erev Yom Kippur: [one who eats on the ninth of Tishrei and fasts on the tenth is as if he fasted both days].
If you look at old manuscripts though there are two other answers (that I'm aware of) offered for the reason he didn't fast on Purim: משום דאתעביד ביה ניסא because a miracle happened that day, and דמעיקרא להכי איתקין because for that reason it was established. On the face of it, the latter answer means that the whole point of Purim was for feasting, so you can't fast on it.
Olelot Ephraim (2:304) however writes about the latter reason:
וכל מאמרים אלו דרך אחד להם שיום מתן תורה ויום כפור וימי הפורים כולם יקראו יום מתן תורה. עצרת כפשוטו. יום כפור...שבו נתנו לוחות אחרונות. ימי הפורים כדאמרן... פורים מעיקרא להכי איתקין ר״ל שמעיקרא לא נתקנו כי אם מטעם קבלת התורה וביטול המודעא...
and all these statements are of one theme, that Shavuot, Purim and Yom Kippur are all called a day of the giving of the Torah. Shavuot: for the obvious reason. Yom Kippur: [...] because on that day the second set of tablets were given. Purim: as we explained [on the page before referencing קימו וקבל, Shabbat 88a, etc.]. Purim was "originally established for that", meaning it was established for the acceptance of the Torah [unlike Shavuot which doesn't have a specific date associated with it].
That explanation of that variant could answer your question. (I didn't ask R Schachter if this is what he meant.)
A major problem with this understanding is that there are some manuscripts (brought in Dikdukei Soferim) that give the reason as דמעיקרא להכי אתקין דכתיב יום משתה ושמחה because for that reason it was established as it says "days of feasting and happiness". This indicates the phrase doesn't refer to the acceptance of the Torah.
This is based upon Shabbat 88a which says in the name of Rava that the Jews accepted upon themselves on Purim that which they began at Matan Torah.
Rava said: Even so, they again accepted it willingly in the time of Ahasuerus, as it is written: “The Jews fulfilled, and took upon them, and upon their seed, and upon all such as joined themselves unto them” (Esther 9:27), and he taught: The Jews fulfilled what they had already taken upon themselves through coercion at Sinai.
There is an incredible series of three chassidic discourses from the Lubavitcher Rebbe discussing this subject in great detail starting on page 44 of volume 3 from Sefer HaMa’amarim Meluket.
