All year round for me Bimble...
Ironically i hate the cold weather but commuting gives me the opportunity to keep some reasonable mileage going during the winter months. I dont go out 'pleasure' riding too much when its freezing, but my commutes not long so i can cope with that.
I think the fear of falling in ice is greater than the reality, ive only come off twice in ..i can't remember how many years.
Snow, no problem...this is mine after a commute to work one morning, head down, squinting eyes from the blizzard of snow..
Summer, the hotter the better. Couple of years ago, it was 32 degrees C, i sweated so much i couldnt undress when i got offf the bike