The Kelemer maggid asked: Why is the Day of Atonement called in Hebrew Yom ki-purim ( a day like Purim )?

And he gave this answer: The similarity between the two days is based on the fact that on both days it is customary to masquerade. On Purim Jews masquerade and don the costumes of non-Jews. On the Day of Atonement, they masquerade as pious Jews.

excerpted From "Hasidic Tales and Teachings"
The Yom Kippur Anthology, ed. Philip Goodman
(JPS, 1992)