Hiddenness
The Sages found a hint to Purim in the Biblical phrase ". . . and I will surely hide ("haster astir panai") My face from you on that day. . . " (Deuteronomy 31:18). This is a reference to the hiddenness of the Purim miracles. Since God's name does not appear in the Megilla text, and Purim events appear to unfold "randomly," one could assume that the rescue of the Jews was just "good luck" or shrewd politics. Unlike an event such as the Exodus, when one technicolor miracle followed another, Purim's events are shrouded in the workings of political intrigue. The Sages tell us that this is a message for the future - just as the unlikely events in Purim were orchestratedfrom Above, so too is God at work, even when His Hand is unseen. The mask represents this hiddenness.

On Purim, we dress up in costumes to say things are not always as they seem, and in a minute, they can change. Behind the mask of Nature, God is at work, and although Haman might be wearing a crown this moment, in the next -- it belongs to Mordechai.

This custom of masquerading was firmly a part of Purim celebrations by Italy in the 1500s. Masquerade balls, no doubt influenced by the Springtime carnivals of their non-Jewish neighbors, became popular.

Today, it is children who most of all dress up in costumes. Throughout Israel, children come to school in costume the days before Purim. In parties throughout the country, children are dressed as Esther and Mordechai, not to mention clowns and Teenage Mutant Ninja Turtles.