Purim is a special holiday.  As described in the Book of Esther, in the time of the Purim story, "And the tables were turned, so that the Jews had the upper hand over their enemies." (Esther 9:1)

That is to say, the situation was completely reversed.  Throughout the history of the Diaspora, the gentiles restrained Jews and controlled their fate.  In this instance, Heaven intervened and gave the Jewish people the opportunity to escape the threat of annihilation.

Each year, we mark the miracle of our divine rescue with rejoicing and a festive banquet.  Although this is not the only instance of miraculous salvation of the Jewish nation, the events in ancient Persia were unique in that the state of affairs was reversed so suddenly and so completely.

Dressing up commemorates this aspect of the events commemorated by Purim.  It was as though the nature of the Jew was suddenly transformed; in every direction, no matter what we stretched forth our had to do, we were blessed with success.

Even in our times, this divine blessing continues to accompany the Jewish people at this season of the year.  The entire month during which Purim falls, the month of Adar, is marked by enhanced loving-kindness and blessing from Above.