Why could the thirteenth of Adar not be a day of celebration the very first year?
(9,1) “And in the twelfth month, which is the month of Adar, on the thirteenth day thereof, when the word of the king and his law drew near to be executed, on the day that the enemies of the Jews hoped to rule over them, but it was reversed, that the Jews should rule over those who who hate them.”
It seems that this posuk is coming to teach us that we should not mistakenly think that the decree to annihilate the Jews was anulled immediately when Haman was hanged and that the outcome of the day of the thirteenth of Adar was reversed before the day arrived, and thus it was fitting even then to make the day a Yom Tov. Because the truth is that this was not so - when the thirteenth arrived the decree from Above was still hanging over them, and it was only on the day itself that the decree changed from bad to good.
This is what the posuk is saying: “On the thirteenth day thereof, when the word of the king and his law drew near to be executed”, when the word of the king and his decree was still drawing near to be executed, “it was reversed” - only on the day itself was it reversed “that the Jews should rule over those who who hate them”. Therefore the day remained a normal day and could not be a Yom Tov. But this was only in the first year, but in the following years already from the beginning of the month the whole month was changed to a Yom Tov, as it says in posuk 22 “and the month which was changed for them from sorrow to sadness, and from mourning to Yom Tov”.
Were the rest of the Jews more righteous than Mordechai and Esther?
(9,15) “And the Jews who were in Shushan assembled also on the fourteenth day of the month of Adar, and they killed in Shushan three hundred men but they did not lay their hands on the spoils.”
Behold, when Mordechai and Esther sent out the second set of letters they wrote that the Jews could “take spoils for plunder” (8,11), yet our posuk says that they did not lay their hands on the spoils. Were they more righteous than Mordechai and Esther?!
It seems to me that it was not because they were more righteous, but rather it was because of the teaching of Chazal that one must not serve Hashem in order to receive a reward. But this teaching applies only when one is commanded to do a certain mitzvah, but where one is not commanded it is permitted to do in order to receive a reward, because then he deserves a reward by right, as the sefer Parshas Derachim explains.
Now, the killing of their enemies at that time was part of the mitzvah to wipe out Amalek, and since it was a mitzvah how could they take spoils - that would be considered as doing a mitzvah for the sake of reward! But Chazal teach us that at the time of the events of Purim they exercised their right to claim that they were forced to accept the Torah at Mount Sinai, and thus they rescinded their acceptance of the Torah. Thus at the time when the letters were sent they were not commanded to wipe out Amalek, and thus their action was voluntary and so they were permitted to take the spoils.
But Chazal also teach us that later they voluntarily reaccepted the Torah, and so they were then commanded to wipe out Amalek. Thus they were no longer permiited to do for the sake of reward, and so “they did not lay their hands on the spoils”.