What is the origin of the custom of dressing up in costumes on Purim? Firstly, when the Jews went into exile they were compelled to put on an act, bowing down to the statue of Nebuchadnezzar. On a deeper level, costumes recall the spiritual garments that Mordekhai and Esther merited to receive.

The straightforward explanation of why we dress up:

The customs of Israel are as binding as the Torah. They were accustomed to transform themselves with unusual attire at the time of the festive meal and the rejoicing on Purim, and this is no meaningless behavior. And I have pondered the teaching of our eminent teacher, Rabbi Moshe Hagiz [Jerusalem, 1672–1750], that it is because of what our Sages said: “They did so [that is, bowed down to the idol in the days of Nebuchadnezzar] only for appearance. So too, the Holy One, blessed be He, acted with them only for appearance [as there was a decree issued against them, but it was not carried out]” (Megilla 12a). Therefore, we likewise transform ourselves, with unusual attire, at the time of rejoicing, making our appearances unrecognizable. This is a precious explanation of the straightforward meaning of the custom.

An additional explanation for the custom of dressing up:

But I will tell you that which is in my heart…the customs of Israel are as binding as the Torah. They were accustomed to dress in unusual attire at the time of rejoicing to remember the miracle and the wonder that God performed for Mordekhai and Esther, that they were dressed in precious, spiritual garments, those which saintly people wear in the Garden of Eden. (Rabbi Tzvi Elimelekh Shapira, Benei Yisaskhar, Ma’amarei Ĥodesh Adar 9)

Further reading: For more on the Jewish people’s insincerity when bowing down to the idol, see A Concise Guide to the Sages, p. 299.

The Gerer Rebbe, writer of Ĥidushei HaRim, once said, at a Purim banquet: “When we begin reading the book of Esther, we are astonished by the fact that it is filled with stories about a Persian king: It tells of the feast he arranged to mark the third year of his reign; it describes Queen Vashti, who did not want to come before him; it outlines the arrangements involved in selecting a new queen. And all the while, we are wondering: What does this have to do with us?

“Only at a much later stage in the megilla do we suddenly understand that all the earlier incidents were part of a chain of events that resulted in the miracle of Purim.”

When the saintly man had finished speaking he was silent for a moment, deep in thought. Then, he stood up and said: “The messiah will arrive in the same way. Various unusual events will occur, and then, all at once, everyone will understand that they were all connected to the redemption.”

It is better for a person to increase his gifts to the poor than to enlarge his banquet or the portions sent to his friends, as there is no greater or more glorious joy than that of bringing joy to the hearts of poor people, orphans, widows, and converts. (Rambam, Mishneh Torah, Hilkhot Megilla VeĤanukah 2)

The Purim miracle led Israel to experience gratitude, and as a result, they merited connection to God. (Rabbi Eliyahu Eliezer Dessler, Mikhtav MeEliyahu, vol. 2, p. 134)

Through the Purim feast, we will be filled with God’s blessing…and it is a glimpse of the feast in the World to Come. (Rabbi Tzadok HaKohen of Lublin, Likutei Ma’amarim 141)

Further reading: For more on how to fulfill the mitzvot of mishlo’ah manot and gifts to the poor, see A Concise Guide to Halakha, p. 265.

Further reading: For more on Passover and the exodus, see A Concise Guide to the Torah, pp. 135ff, 161, 315, 471; A Concise Guide to the Sages, pp. 84, 306; A Concise Guide to Halakha, pp. 274ff.