Fighting depends on Weather, not season.
If there's enough rain, most armies went to camp, even if there was no snow, after all, because slogging through mud could cost you horses, wagons, and supplies. High winds could foil all but the heaviest siege weapons, too, rendering archers and other ranged attackers functionally useless.
In adverse weather, you hunker down and wait for it to pass. If the adverse weather is expected to last for weeks or months, you turn your armies around and go home 'til it's over.
In D&D, though? One druid of 15th level or higher solves -all- your logistical problems with a quickness. Sure, your soldiers may need to trod snow to pack it down for the wagons, but the 5 mile radius of Control Weather means there are no blizzards, no rainstorms, and no high winds when it's time to fight.
They can't fix Mud, at that level, though. So if it's a particularly wet region with thick mud you're SoL.