Hi Ribzi,
Welcome to the forum. I think that you will appreciate that starting from Leon in December you will be walking into winter. You will already be at a good height above sea level as you start and then beyond Astorga you will be rising to approx. 1500 metres. Galicia, which is where you are heading, is beautiful but the weather is not kind to pilgrims; it is also known as 'Green Spain', i.e. its wet, and in the mountains you can also expect snow - I haven't been there in the winter but I'm sure that if you check out the weather reports you will find snow; they even get it in Spring and Summer. So weatherwise I would say prepare for the worst.
Also, while there is plenty of accomodation on the route, make a thorough check of which refugios, hotels, etc are open in the winter, even to the point of checking those places/villages where, initially, you don't intend to stay - 'the best laid plans ......'.
That's all for the moment, hope you have a great trip.
Buen Camino,
Brendan