Harvest. Maybe summer magic is about harvesting things in their natural time and then feeding people (and animals, and the compost heap) with them. You know, the great circle of life thing.

In which case, winter magic might be an attempt to stop the wheel. Freeze it, "save" it - can a thing be saved if it is never used? What if it's intended to be used, but is instead frozen and hoarded, taken out of the cycle?