What's the first appearance of Becoming the Costume that happens on Halloween?
Something turns people into whatever costume they are wearing.
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Great and interesting question!
From the looks of it, the first instance of Becoming the Costume is
The Twilight Zone (1959) has "The Masks", where a dying millionaire forces his worthless heirs to wear masks caricaturing their worst personality traits - if they take them off before midnight, they get cut out of his will save for some fare to take the train home. At midnight, he dies, and they shed the masks... and discover their faces have taken on the shapes of the masks permanently save the good-hearted millionaire. He retains his human face. It should be noted this isn't set on Halloween, but Mardi Gras in New Orleans.
Although not set in Halloween as requested, it may have inspired other occurrences like:
The House That Dripped Blood: In "The Cloak", actor Henderson purchases a vampire's cloak to add verisimilitude to role as a vampire, only to discover he transforms into a vampire whenever he dons the cloak.
The House That Dripped Blood is a 1971 British horror Anthology Film directed by Peter Duffell and distributed by Amicus Productions
The TV special Witch's Night Out (1978) has a witch turn two kids and their babysitter into the monsters they're costumed as. When the adults find out and turn on her, she explains that the point of Halloween is to try out different personas and imagine your potential. The adults happily start calling out requests for one-night transformations themselves.