Mountains are different but on the east side of the Rockies, winter is the BEST time to fly! Summer - bleah...this week Denver will hit 100. Rare, but it happens. With days in the 90s, forget it. No, I'm not going to get up at the crack of dawn just to fly. I despise hot, miserable summer in my flying convection oven masquerading as a cherokee. Annual scheduled for July, every year.
Winter winds around here are very mild unless it's a snow storm. But you're not going to fly in that anyway. If there's no wind, and the sun is shining, the temp isn't an issue. I've shoveled snow in ski pants and a t-shirt in 0F temps, felt like it was 60. Ski pants only because the snow was 3-4 ft deep. Winter flying in Colorado - boots, heavy jacket, warm pants to preflight. Take off the jacket (but keep it handy) before takeoff but by the time you're at 2000 AGL its 70 inside and below zero outside.