Spain will most definitely be cold during December. However, just how cold depends on the region in which you visit. You can expect Madrid to be particularly chilly with highs around 50°F/10°C and lows in the high 30s (Fahrenheit).
In San Sebastian, on the northern Atlantic coast, it's a little warmer at around 55°F/12°C, but if you want to squeeze the most sun and warmth out of Spain's winter you'll head to the Mediterranean coast. Barcelona experiences highs of around 60°F/15°C, and those temps slightly increase the further south you get down to Andalusia.
Crowds & Costs
December might be the most packed month during the offseason. This is because everyone, locals and visitors alike, are getting into the festive Christmas spirit. Then there's New Year's Eve, which comes with its own raucous celebrations. Compared with the offseason, flights and hotels will be a bit more expensive during most of December, but prices really spike during Christmas week.
Where to Go
Spain is so vast, and regional climates so disparate, you can plan any type of winter holiday you'd like. If winter sports are your thing, there are options on all sides of the country that have you covered (more on this below). If you don't ski or snowboard, you can opt to stay mostly indoors and visit the many world-class museums in the country. These include the Prado in Madrid, the Picasso Museum in Málaga, MACBA (Museu d'Art Contemporani de Barcelona), and the Guggenheim, in Bilbao.
On the other hand, if you're desperate for sunshine, then visit the Canary Islands. While it still might be a little too chilly for swimming on Gran Canaria, the sun will likely be out and you might be able to get a bit of suntanning done on the beach. You should also visit Barcelona and the capital of Andalusia, Seville. Not only will you probably get some sun, but these cities also have great Christmas markets during the month of December.
What to Do
While Andalusia is typically known for its warmer temperatures, the Sierra Nevada Mountains, near the southern coast, are prime skiing/snowboarding territory. December kicks off ski season in Granada, which sits at the foot of the Sierra Nevadas. The Sierra Nevada Ski Station is the premier resort here, featuring 65 miles (105 km) of ski runs.
If you don't plan on traveling that far south but are still interested in a ski holiday, you can head to northeastern Spain towards the border with France. A good destination is Zaragoza, the capital of Spain's Aragon region and a city abounding with baroque and gothic architecture. It's also the gateway to the Pyrenees Mountains, and there are a number of ski resorts within a manageable drive of the city, including Aramon, Formigal, and Panticosa. Aramon is the largest, with 160 miles (259 km) of ski runs.
Events in December
Christmas Day. Dec. 25th is a relatively understated affair in Spain. The real celebrations and unwrapping of gifts occur on Epiphany/Three King’s Day, on Jan 6th.
New Year's Eve. Dec. 31st is party time in Spain. Friends and family typically get together for dinner before watching the clock strike down to midnight. When it does, tradition dictates that you must eat 12 grapes, each in unison with the chiming of the clock, which portends good luck and prosperity for the upcoming year. Many folks congregate in plazas for the event, and one great party is always at Puerta del Sol Square, in Madrid.
Traveling to Spain in December? Check out these great itineraries.
Seville, Madrid & Barcelona - 7 Days. The trip starts historic Seville, continues in Madrid, and finishes in cosmopolitan Barcelona. Make sure to be in Madrid for the great party on New Year's Eve.
Ultimate Canary Islands Tour - 8 Days. This exciting itinerary covers the archipelago's three easternmost islands with the help of a private guide. It's the perfect destination to get away from the gloomy December weather on the mainland.