In my opinion, you can visit Jura anytime during the year. During last decade the winters in Poland are rather mild with not a lot of snow and ice. However, it might happen that for short period of time there is a lot of snow or that temperatures drop to even -20 Centigrade. Although most of the attractions are open all the yearlong; some of them might be closed for the winter time. If you are particularly wishing to see / experience something and you are planning your trip between November and March, please double check if the place of your interest will be open during this time.
There are however some attractions available only during the winter; for example, ski-lifts and slopes. For amateurs of winter time and sports there are also a few cross-country ski trails. More information about them you can find here.
If you are not a skiing fun or you rather prefer summer activities, I would definitely recommend visiting Jura in late spring or during autumn. May, June or September, October might be the best choice. July and August are also fine, but there can be a really hot periods during these two months (with temperature over 30 Centigrade) and it can be quite crowded around as these two months are school holidays’ period in Poland.