I booked a holiday in Andalusia around Christmas a few years ago
the weather was awful. I enjoyed my trip anyway... even I would had love having some warmer days... like my family and friends who stayed in Barcelona having milder temperatures and sunnier days than we had in the South!!!
it all depends on your luck, but expecting "good weather" anywhere in mainland Spain in December is wishful thinking.
as said above, only the Canaries can guarantee "good weather" (and not all the islands)
great tapas and seafood and jamon... you will find these all over the country
and you will not find many tourists around on these dates, anywhere, except at the "must sees" of the most popular cities
