Fall for me as well, there is just something about Fall that simply screams imagination and I dunno, there is just something unique about Fall.
It also on a more grounded level means the time of gathering around inside, after having fun in the leaves, getting some hot chocolate and taking out the games.
I voted spring. For some reason, I like to begin my campaigns in spring. I think it's because spring has connotations of new beginnings and new possibilities, which I like to evoke in my games.
I voted summer...maybe it was the fact that when I was a kid I spent a good portion of it reading and playing D&D...besides, fall and winter are reserved for my WoD games...Started my Changeling campaign up already.
Fall is a wonderful time for gaming. It reminds me more of Ravenloft than D&D in general though.
To me, summer is more D&D-esque than fall. It's warm and easy to spend an entire day outside exploring the wilderness, even if the "wilderness" doesn't have goblins and dragons wandering about. I love it being hot enough to sleep outdoors.
Not only I have I started all my Campaigns during the Fall season of the game world, I've actually begun them in the Fall of the real-world. Probably it goes back to starting games at the beginning of the school year. There may also be a Led Zeppelin influence by way of "Ramble On" (the bit about it being time to go and the Autumn moon lighting the way). In any event, I always have a Halloween-themed game early on in the Campaign as well.
Winter -- I have my best D&D memories of huddling in one of my best friends' Antebellum house in the freezing cold, in front of a fireplace, bundled in a blanket and light coat, over Christmas break, figuring out how to make the most kick-butt character possible with the AD&D rules, and talking about the most nonsensiical stuff from Star Wars, to Terminator, to AC/DC, Metallica, and just how little Jack Daniels it took to make one of our friends drunk.