Dressing up in outrageous costumes is one of the best known customs of Purim. The first mention in halachic sources comes from the 13th century. Rabbi Kalonymus from Provence writes that Jews have a custom of dressing up as gentiles and as the opposite sex.
But why do we do it? Two of the Purim themes are reversal (Esther 9:1: “The opposite happened and the Jews had their enemies in their power”) and hiddenness. Esther conceals her Jewishness until she needs to beg for the lives of her people.
The Talmud teaches that Haman’s plot was a punishment for the Jews’ participation in debauched royal banquets. Jews were becoming too at home with the crudest elements of their surrounding culture and forgetting who they really were. Dressing up and playing with our identity on Purim can help to us reach a deeper understanding of the hidden miracles of the festival and of who we are at the deepest levels of our selves.
Rabbi Julian Sinclair