Hola Cornelio,
Travelling in December is a delight, all the lovely decorations and good cheer. I was in Galicia for Christmas last December and it was chilly, in the mid 40's, but warmer than Madrid, which was snowing when I left on the 24th, and 37ºF. Galicia is a bit drizzly, and if you go to Santiago, you will see the "llovizna" a drizzly, misty type rain, that wets your hair but does not permeate your clothes as much. It does wonders for your skin!
I was surprised with the weather, it was much milder than I had expected. I found it very comfortable, overall, and I'm from North Florida, which is somewhat like South Georgia!
Do bring a coat, hat and gloves. I brought an umbrella, but never used it.
Have a wonderful viaje!