Weather in Spain during the Winter (mid-late December) ?

If you want pleasant weather in mainland Spain at that time of the year, you should really stay on the southern coast and lowlands. Even there it's not guaranteed. Some places in Spain are ice-cold and snowy, others are too damp and rainy. In Seville or Málaga, for example, it will probably be mild and you may get some nice sunny days.

Well, the problem is you take 3 months which are very different, as it ranges from late summer through Autumn and into early Winter so your question has no real answer. The weather will vay a lot in those months.

In September you can swim in the sea in the Med Coast even until early-mid October but it's too cold in November-December for my taste, some Northern Europeans still swim in winter, though.

You can get some rainy days, of course, but it won't be raining non-stop. The Mediterranean has a mild winter, so in general you'll have nice temps during the day and colder in the night. December won't be dreary and chilly, but it won't be spring like. Late September is a nice time to visit, warm and nice, no longer terribly hot, but you could still get some rain occasionally.

Seville? Quite dry and never too cold, a good option. Could be a bit too warm in mid September, though.

The Canary Islands are the safest best, as you've been told already.

It's damp and chilly and dreary, though there are sunny days. Depends where you go.

There are 17 regions. You really want US to do all this work for YOUR vacation?

Know this: If your purpose is to get sun, you probably won't. If your purpose is to visit monuments...hours are shorter, but lines are almost non-existent, and things aren't crowded.

Weather in this period is too iffy to bank on.