Sows give birth in the den so they will den up based on when the cubs should be born, weaned and how old they should be when they emerge in the spring. Timing is more strict for sows.

Boars will stay out as long as they can find food. When we have a banner beech nut crop, boars will stay out into mid-December. Regardless of how cold it is. I imagine 3 feet of snow would end the party early no matter what other variables.