winter is a poor time to try to march and fight - but it was still done from time to time.
One shouldn't forget that there were also a number of benefits during winter. For one, roads tended to be in good condition as the cold made them hard - compared to the mud of spring and late autumn. And many rivers normally only fordable at a few strategic places could become generally passable. A wintercampaign would however need more preparations and adequate equipment (good clothes and shoes), but it could be still be quite advantages to not sit idle during the whole of winter.