Blue Graduation Cap

Hardmode plus a waning crescent overhead is what makes the Clothier list it. Look up at night before you walk to his house: if the lit sliver is thinning down toward a new moon, the cap and gown are in his shop that evening.

There is no item or action that shifts the phase, so a full lap of the cycle can pass between chances. If more than one player wants the outfit, buy the extras during the same night rather than banking on catching it again.

Cap and gown go in the social slots and leave your armor alone. The gown reaches far enough down that a separate leg piece is pointless, so half your vanity slots stay open for something else.

The black and maroon sets are the same outfit in different colors and sell on different nights, which is the only reason to care which phase you caught.