Elf Hat

The whole set comes off one enemy, the Zombie Elf, and that enemy only appears while the Christmas season is running in a Hardmode world. The Frost Moon is where they arrive in numbers, so the sensible way to build the outfit is to run the event and let the ordinary spawns pile up while you work on the bosses.

Each piece rolls on its own and the chance per kill is thin, so one event usually leaves you a piece short. The season follows the calendar, which means an outfit you do not finish in December waits a year unless you move your system clock.

Hat, shirt and pants go in the social slots and change nothing about your character. The set went into the game before there was any way to get it and sat in the files for a couple of updates before the elves were given the drop, so worlds from that period never saw it.