Opossums do not hibernate.  Winters are a dangerous time for them. Opossums are not equipped for harsh winters. They sleep in hollow logs, under porches, sheds or in chicken coops. They do not dig but will fill their area with leaves and sticks for warmth. During a storm they may hunker down for a day or two but they will be looking for food soon after. In fact, only about 30% of opossums survive their first winter. They freeze to death or starve.  They have no fur on tails, ears or feet and frostbite is a real threat. Frequently the tail becomes infected causing death.
In the northeast we release no later than mid-October to give them time to acclimate to the environment and to find food sources and shelter.