Assume roughly a third to a half of seasonal businesses will be shut, and plan around the ones that are not. The reliable openings are almost always the same: bakeries, bars, hardware shops, pharmacies and whatever the town's residents actually use.
Two practical habits make the difference. Confirm your accommodation by email rather than through a booking platform, because winter closures often fail to reach the platform. And buy food on Saturday — Sunday closing is near-total in most of rural Europe out of season.