It seems this festival was one where the people and kings swore oaths. It was a big time of gathering at Tara in Ireland.
All sorts of disputes were settled at this time, I guess because the weather allowed people to gather. It does not seem to be a harvest festival, as the harvest comes in a little later. It was more a time of bargains and settling disputes, maybe there was good old red neck Celtic fist fights, and people threatening each other with pointy sticks, this could also be a time when chieftains and kings were sworn in, and when the kings has sex with horses in front of the crowds. In this way the king could give fertility to the land and entertain everyone at the same time. After this the king was always known as the Horsefudger. Seems to not have a thing to do with the sun or farming. More like a time when the courts were in session and you could bargain for that horse that the king just had sex with, or argue over who had the right to have sex with who, or where the meets and bounds of land was. It was also a time when Celtic motorcycle gangs gathered to swear allegiance to their lord, thug, so they could enjoy being thugs and riding motorcycles and beating the stuffings out of anyone who insulted the chieftain or burnt a faery flag. Just good old Celtic fun, and you could also buy a pointy stick from the pointy stick salesman, because nothing will scare the hell out of the romans like a pointy stick.