Greg used to do it based on when it reached a certain temperature at his house (don't remember what it was), then he switched to when the last lift stopped spinning in the east a year or two again. Then it would go back blue again when he got his first frost at his house. As SM mentioned he also changed the order of the forums, so that the non-snow based forums were at the top for the summer.
He also used to take pleasure in making sure that everyone saw the change, so sometimes he'd force everyone to the default theme when he switched to green. Just to make those of us who don't like the green suffer for that first time we logged in after the change.
You can see what the green "summer" theme is all about by going to the bottom of the page and changing the drop down box on the left from "Winter" to "Summer".
Yeah I toggled it before. So what's still out there and open? Sugarloaf is about it?