FROZEN ground is hard to break.
I have seen building construction start in mid February in Minneapolis when the demand is there. Excavation and foundation digging can be done in almost any weather. The Cold and snow has little effect on large excavators.
These are seasonably adjusted numbers. Meaning they are manipulated to look better than they actually are.
A backhoe has no problem digging up several inches of concrete, It has less issues with several inches of frozen ground. The only thing that isn’t done in very cold weather is new site concrete pours. That doesn’t stop new starts.