A typical winter day would be a high of -6 Celcius and a low of -20, but there would be cold snaps of -20 or colder. Winterization itself was several things.
1. In late fall we'd make sure each colony had enough honey to fuel them through to spring (a quick lift would tell you). If short, we'd put sugar saturated water in a tray on top of the colony. The bees would move the sugar into the colony and a couple days later we'd take out the bone dry trays. Failing to ensure enough fuel meant certain death for the colony, though for some in the trade the math was that it was cheaper to buy nukes (a colony nucleus of a queen and some workers) in the spring. Our math was that We liked to have strong colonies in the spring to sell nukes.
2. A bee colony is basically a rectangular box sitting on a frame. We had rectangular insulation that stored flat but easily expanded to slide over each colony before the first snow. The colonies would get buried in snow, which was excellent extra insulation.
3. The bees themselves did the work to survive the winter. They'd huddle in a ball, burning honey to generate heat (a bee could heat itself to something like 40 degrees C), fanning their wings to spread the heat. The bees in the centre of the ball would move out to the periphery while those on the periphery would move into the center.
A cold snap that lasted too long was a disaster as the bees would tighten the ball for greater warmth and then run out of honey within the ball. Those colonies would die. In the spring you'd find the tightly clustered ball of bees, dead, surrounded by honey not that far outside the ball.
You needed at least one brief warming period in a cold snap in which the ball of bees would expand, find a new patch on unconsumed honey in the hive and then recontract around the honey.
If we did our work properly in the fall, we'd have 90% or more of our colonies make it to spring, most strong so we could make nukes to replace our losses and sell on the extras.