I can clearly recall, some time around 98 when there'd been 2 festivals on the trot that were muddy, Michael Eavis declared in an interview that the reason he held it on the week he did was that the nights were shortest close to the solstice and that farmers almanacs going back centuries showed it to be statistically the most likely to be dry of all the weeks of the summer. I thought that was pretty sound reasoning, backed up by evidence.
Don't forget: Beyoncé has something up her sleeve for Tuesday...
https://eu.usatoday.com/story/entertainment/music/2025/01/13/beyonce-surprise-announcement-january-14/77602734007/