It’s funny the way they couldn’t just say hey, today is halfway between the solstice and the equinox, and from now on the sun’s going to be getting weaker; and also it’s the beginning of the harvest season, so let’s have a big festival!
But of course it’s a lot more complicated than that. There was this Celtic sun god named Lugh whose foster mother died of exhaustion after being forced to turn a forest into a wheat field, and also there was a saint named Catherine who was supposed to be martyred on a wheel (don’t ask) in Alexandria; but she made the wheel disappear so they had to cut off her head instead.
So Lugh’s funeral games for his mother turned into the first harvest festival, and then the Christians as usual took over and made it St. Catherine’s Day for awhile (until they moved it to November), and so at this Lugh nasadh (festival) they set tar-covered “St. Catherine” wheels on fire and rolled them downhill, but that seems pretty pagan, so they changed the name to Lammas, loaf-mass, and you’re supposed to bring a loaf of bread to mass.
Syncretism again. It’s very complicated, trying to turn pagans into Christians.