Camino Frances in early Dec, rainy?

In December in Galicia you might find sun, hail, snow, wind, fog and rain. Quite possibly in the same afternoon It really is highly unpredictable and the only practical option in my opinion is to assume that there will be heavy rain and go well prepared: waterproof jacket (and probably trousers) and an adequate change of clothes kept safely stowed in waterproof bags. Not wise to assume that you will always be able to dry wet clothes thoroughly by morning.