We are hoping to spend the first 3 weeks of December in Spain. Arriving in Barcelona and going to Grenada, Seville and who knows. Leaving from Madrid. What's it like to travel in Spain in December? Any suggestions for an itinerary? We've never been to Spain.
I traveled to Spain last December. It was pleasant, mostly sunny, with occasional rain, and crowd free. I wouldn’t hesitate to go in December again.
Some people would consider much of Spain to be on the chilly side in December. Of your list, Madrid would be the coldest, flirting with freezing overnight. (If you live somewhere colder, it could be a non-issue.) Also, note that the days are short—Madrid and Sevilla are on the same latitudes as NYC and Richmond, VA, respectively. Though being in the wrong timezone for its geography, sunrise, and sunset are both later than they would be in the US.
Spaniards flock to their pedestrianized city centers for Christmas shopping, which can become quite crowded on weekend afternoons. It's mostly festive and social, but for me, in Madrid, those crowds can be a little overwhelming.
On the rough itinerary, it usually makes more sense to fly from Barcelona to Andalucia. From there you can tour and return to Madrid via trains and busses.