There are several reasons given for the age-old custom to dress up in costumes and wear masks1 on Purim. Here are some of them:

The Hidden Miracle

In contrast to the overt miracles of the holidays of Passover, Chanukah and other Jewish holidays, the miracle of the holiday of Purim was disguised in natural events. Here is a sampling of the story: The king wanted his wife to come to a party; she did not want to, and she was killed. Then an evil man wanted the Jews dead and plotted to accomplish this with the approval of the king. The king remarried, and his new queen happened to be Jewish, and arranged for the decree to be countered. Only after the fact, when one looks at the entire story, does one realize the great miracle that transpired.

The custom of wearing costumes on Purim is an allusion to the nature of the Purim miracle, where the details of the story are really miracles hidden within natural events.2

Only Pretend

The Talmud writes that just as the Jews at the time pretended to be serving other gods, G‑d pretended that He was going to destroy the Jewish nation, and in the end He did not.3 Rabbi Tzvi Elimelech Shapiro (1783–1841), known as the Bnei Yissaschar, writes that this is the reason we pretend to be someone else on Purim, since both the Jews’ and G‑d’s actions were masked by other intentions.4

Avoiding Shame

We dress differently on Purim to minimize the embarrassment of the poor who go around collecting charity on this day—a day when we give charity to everyone who outstretches their hand.5

The Royal Garb

To commemorate the dressing up of Mordechai in King Ahasuerus’s royal garments in the story of Purim.6

For more, see Masquerade!, 5 Quick and Easy Purim Costume Ideas and our additional articles on Dressing Up on Purim.