Modern fiction about fairies often describes them as being divided into Seelie and Unseelie Courts or Summer and Winter Courts, sometimes Spring, Summer, Autumn and Winter Courts.
A lot of people seem to believe that this is an established part of fairy legend - when I'm talking to fans of Gothic "real faeries" (as opposed to the child-friendly Enid Blyton kind) they often mention this. But I'm not sure that it actually is any older/more authentic than those things.
Wikipedia says that the idea of Seelie and Unseelie Courts does occasionally appear in Scottish folktales, and that they may have got the idea from Norse mythology, which does, at least sometimes, divide its "elves" ("alfar") into "light elves" and "dark elves". "The Seelie Court" is mentioned in the Scottish ballad Allison Gross, and according to https://britishfairies.wordpress.com/2021/01/24/the-seelie-and-unseelie-courts/ the first record of that ballad is from 1783 and it may be older still.
The Summer and Winter Courts, though, I haven't from a quick look been able to track down any mention of at all other than modern fantasy fiction. I've heard that there's a scholarly theory that the two rival "kings of Annwvyn" in the story of Pwyll Lord of Dyfed in the Mabinogion were meant to represent winter and summer (it's mentioned in the introduction to my copy), but that's only a theory and is not in any way spelt out in the story.
Is there any reference to Summer and Winter Courts in any old folklore?