This is how I winter my bees. I learned this from attending the Univ of MN Beekeeping Short Course 25 years ago. It worked then and works to this day.
Now we have mites to contend with. There were no mites 25 years ago.
I usually put my covers on around Thanksgiving. If we are slated to get several inches of snow before then, I put the covers on before it snows.
Once there is snow on the ground it can get quite cold and the bees need the covers on by then.