Fall > Winter > Spring > Summer
The scorching summer heat is basically like being roasted alive. It sucks. Can't really do anything about it except rely on other methods (air conditioning, cold drinks, the beach), and even then, those are pretty much temporary solutions. The only thing it has going for it is
STEAM SUMMER SALE. PRAISE LORD GABEN.
Spring is okay, it kind of steps into high temperatures sometimes, but it mostly stays warm. I don't get spring allergies, so it's not all that bad for me. Not a lot of fun holidays. The only one I could think of is Easter and Spring Break.
Winter is great, and the cold weather is fantastic. You could always just put on more clothing or blankets if you're freezing to death. You could also warm yourself up with hot drinks, like hot cocoa or hot tea and watch it snow outside your house. Of course, there's the problem of having to shovel the snow and slipping on the ice, but that mostly depends on how kind and considerate your neighbors are. For me, some of them are too lazy to pour ice onto the sidewalk or even shovel their share of snow. I usually have to cross the road if their sidewalk is a 4-feet tall pile of snow. Holidays include both Christmas and New Year's, which are pretty close to each other. Great time to get games/gifts. Speaking of which.....
STEAM WINTER SALE. PRAISE LORD GABEN.
Fall is the best, it's a fantastic mixture of not-melting-to-death and not-constantly-slipping-on-ice. You could also rake leaves and jump into the pile, if you want. There's Halloween and Thanksgiving, if you're into that.