http://revach.net/article.php?id=4423

Section: Moadim   Category: Purim
Purim: Meshech Chochma - Klal Yisroel Rejoices In A Different Kind Of Victory
The days we celebrate Purim seem to be an odd choice.  A more logical choice would be to celebrate on the 13th, the day we defeated our enemies or on the day that Haman was hung.  Why do we celebrate on the day after Purim the day after the war?

The Meshech Chochma in Parshas Bo answers that this teaches a great lesson about the goals and the victories of Am Yisroel.  We don't fight wars to take revenge and we don't rejoice in our enemy's downfall.  Our simcha is that we are free from the things that constrain our Avodas Hashem and harmful to our existence.

There is no great joy for us on the day of the war.  We do not commemorate the great victory in battle.  We celebrate the day the war was over and we were Mikabel the Torah with renewed vigor.  Am Yisroel Chai.  Am Yisroel lives on by the grace of our Father in Heaven who we can serve with joy.  That is true cause for celebration.