Coldest actual temperature was probably in the minus 10 range. Coldest windchill was minus 25. It was pretty brutal, but I was a lot tougher back then. The wind was a real howler that November morning, but I managed to tag a small 10 point about an hour into the hunt. I immediately got down from the stand and went to where the buck was when the arrow struck. In a matter of maybe five minutes, the blood trail had already frozen pink. It honestly hurt exposing my hands to the cold during the field dressing. But it was worth it!
Wear as many layers as possible, as long as you can still shoot an arrow. I had a number of layers inside a set of insulated coveralls that morning. Heavy facemask, a couple of warm caps, good insulated boots and chemical handwarmers in each pocket. And don't walk to your stand wearing all these layers, carry some in.
On really cold mornings, I'll also get to the stand at pink light, rather than a half hour or so, early.