Maroon Graduation Cap

Two conditions have to line up. The world needs to be in Hardmode, and the moon has to be in the right phase, so a shop visit on an ordinary evening shows you nothing unusual. Phases move on by one each in-game day, which means a miss costs you a full cycle of waiting unless you push time forward with an Enchanted Sundial. The other graduation colors work the same way on phases of their own, so which gown you can walk away with is decided by the sky rather than by you.

Cap and gown, with no leg piece, so your own pants stay visible underneath and the look holds up best over something plain. Both go in the social slots and change nothing about how your character performs. The gown covers most of the body, which is handy in early Hardmode when your real armor is a mismatched pile of ore. Beyond the joke value it is one of the few outfits built for a scene rather than a character, so it earns its keep in a schoolroom build or a group shot with friends.