It is believed the earliest mention of wearing a costume at Halloween dates back to 1585 in Scotland; however, the practice was most likely common place before then. The tradition of costumes originated with the ancient Celtic festival of Samhain when people would light bonfires and wear costumes to ward off ghosts. Wearing costumes on All Hallows Eve, aka Halloween, came from the belief that supernatural beings, or the souls of the dead, roamed the Earth on October 31st. People dressed up to impersonate the dead and go house-to-house reciting verses or songs in exchange for offerings on behalf of supernatural entities. Impersonating these beings was also a means of protecting oneself from them.

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Credit - https://webbweekly.com/articles/2020/10/21/the-history-of-halloween-costumes/

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