That is why Jewish villagers who live in un-walled towns celebrate the fourteenth day of the month of Adar as an occasion of gladness, feasting and a Yom Tov... Mordechai recorded these events and sent letters to all the Jews... They were to observe them as days of feasting and gladness...
The Gemara in Megillah (ה ע"ב) points out the following; מעיקרא כתיב שמחה ומשתה ויום טוב ולבסוף כתיב לעשות אותם ימי משתה ושמחה ואילו יום טוב לא כתיב - At first, when the Jews initially celebrated the great miracle, it is written ‘Gladness, Feasting and a Yom Tov’ but later, when Mordechai was establishing the celebration of Purim for the future generations, it is written, ‘They were to observe them as days of feasting and gladness’, while the term ‘Yom Tov’ is not written. This seems to imply that, initially, the Jews of that generation, after having experienced the miracle, had spontaneously accepted upon themselves to celebrate the great miracle of Purim as a Yom Tov, in addition to celebrating it as an occasion of gladness and feasting. Nevertheless, this idea that Purim should be celebrated as a Yom Tov was ultimately rejected, and thus when Mordechai established the celebration of Purim for all generations, he did not establish it as a Yom Tov, rather only as an occasion of gladness and feasting.
We need to understand why the Jews didn’t see fit to establish the day of Purim as a Yom Tov for the future generations, although initially they did see fit to establish it as such.
The Gemara in Shabbos (פח ע"א) tells us of a superior virtue which was gained from the great miracle which occurred on Purim. The Gemara brings the following Aggadic teaching about what occurred with the Jews when they were standing at Har Sinai, about to accept the Torah.
ויתיצבו בתחתית ההר. אמר רב אבדימי בר חמא בר חסא מלמד שכפה הקב"ה עליהם את ההר כגיגית. ואמר להם, אם אתם מקבלים התורה מוטב, ואם לאו שם תהא קבורתכם. מכאן אמר רב אחא בר יעקב מודעה רבה לאורייתא. אמר רבא, אף על פי כן הדר קבלוהו בימי אחשורוש, דכתיב קימו וקבלו היהודים, קיימו מה שקבלו כבר. - The Passuk says, ‘They stood at the foot of the mountain’... This Passuk teaches us that Hashem covered the Jews with the mountain and told them “If you accept this Torah, fine, but if not, your burial will be right here”. Rav Acha Bar Yaakov said, ‘From here stem strong grounds for a notification of coercion regarding the acceptance of the Torah’. That is, if Hashem was to summon them and demand of them why they did not fulfill their commitment to observe the Torah, they could respond that the commitment was coerced. Rava said, ‘Nevertheless, they accepted the Torah again in the days of Achashverosh, as it is written, ‘The Jews established and accepted’, which is interpreted to mean, ‘They established in the days of Achashverosh that which they had already accepted in the days of Moshe’.
The Gemara in Pesachim (סח ע"ב) says as follows, הכל מודים בעצרת דבעינן נמי לכם, מ"ט, יום שניתנה בה תורה הוא - All authorities agree that in the case of the Yom Tov of Shavuos one must devote at least part of the day to himself, by eating and drinking, for it is the day on which the Torah was given, and by feasting on that day, one demonstrates that he rejoices upon having received the Torah and doesn’t regard it as a yoke.
Until the Purim miracle occurred, the Jews had never actually, willingly and happily, accepted the Torah, and thus the Yom Tov of Shavuos carried no special joyous feelings. It was only after the Purim miracle that the Jews felt a special love to Hashem, and willingly and joyfully accepted the Torah. Consequently, the day to commemorate the joyous acceptance of the Torah was not on Shavuos, rather it was on Purim. It was for this very reason that the Jews at first wanted to commemorate the miracle of Purim by establishing it as a Yom Tov, just as the Gemara states regarding the Yom Tov of Shavuos that all agree that it is a Yom Tov that one must enjoy, to show that he rejoices upon having received the Torah. Yet, when it came time to establish the Purim day for the future generations, they realized that although it was only after the Purim miracle that their love to Hashem was great enough to cause them to willingly accept the Torah, nevertheless this love to Hashem, which they gained after the miracle, caused them to joyfully and willingly accept the Torah retroactively from the time that they stood at Har Sinai. Thus, the Yom Tov of Shavuos, rather than Purim, is the time to commemorate the joyous occasion of happily accepting the Torah.
מגילת אסתר אות י"ד
