Weather
With the coldest winter months in the past, March is when Spain begins to open up. The nights may still be chilly, but the more frequent sunny days make the afternoons pleasant. This goes for Spain's northern Atlantic coast, which in winter is hit hardest by offshore rains and storms. While average temperatures in San Sebastián will likely hover in the 50s (Fahrenheit), you might luck out with sunny days that peak in the 60s.
The capital of Madrid enjoys similar weather, but temps dip into the 40s at night due to the capital city's location on a desert plateau. The Mediterranean coast will have the most pleasant weather, with most areas averaging in the 60s during the day and some places reaching daytime temps in the 70s. While the ocean in most of these areas will be too chilly for swimming, the water in the far south of Andalusia might be the exception to this rule.
Crowds & Costs
If your goal isn't to spend all your time on Spain's beaches, March might be the best month to visit. Half of March is still winter, so the crowds are thin, and flight prices and hotel rates are often lower than they would be in the shoulder season of April to May. Count on saving 50% or more on flights and between 20%-30% on hotels, whether you're visiting Madrid, Barcelona, or any other major city.
Where to Go
To enjoy the most sun, visit the Mediterranean coast. There will likely be some rainouts in the north and the region of Catalonia, but you should get lucky with good sunbathing weather, at least occasionally. It will be increasingly warmer the further south you travel, particularly if you head down to Andalusia. Historic cities like Granada and Seville make for great visits in March, as do Málaga and Cádiz. If you're traveling on the coast during the middle of March, visit Valencia (more on this below).
For those arriving outside Europe, there's a good chance you'll be arriving in Madrid. March is a great time to kick back and enjoy Spain's capital, as there's much to do in the form of culture, cuisine, and nightlife. Making a combined Madrid/Mediterranean coast trip an attractive proposition during this month.
Chat with a local specialist who can help organize your trip.
What to Do
Culture, music, parties, and food are in abundance all year in Spain. As for culture, if you find yourself during a rainout in Barcelona, stay in and visit the city's premier museums, like the Picasso Museum and MACBA (Museu d'Art Contemporani). You could do the same in Madrid (the Prado Museum) and the northern Atlantic coast (the Guggenheim in Bilbao).
March is also a good month to see Spain's famous sites. Naturally, this includes paying a visit to Gaudi's gothic masterpiece cathedral, La Sagrada Familia, in Barcelona. If you are in Andalusia, visit the medieval mountaintop town of Ronda, with its famous stone bridges. And in Madrid, if the weather is nice, you can stroll around the 19th-century El Retiro Park amid its many fountains, lakes, and green spaces.
As for the party, there's none bigger than the Fallas Festival, located in the city of Valencia on the Mediterranean coast. It's a raucous and wild happening that involves plenty of drinking, dancing, and awe-inspiring pyromania. To be on the southern coast of Spain during this time and not attend Fallas would be a great travel faux pas indeed (more info below).
Events in March
Las Fallas, Valencia. The city of Valencia comes aflame from March 15-19 during one of Spain's most famous festivals. For this celebration of St. Joseph, the city engorges with revelers drinking and dancing as massive ninots (wooden/paper-mache sculptures and floats) parade down the streets in great processions. These are complemented by fireworks and bonfires, culminating in the ceremonial burning of the ninots on the final day.
Traveling to Spain in March? Check out these great itineraries
Best of Barcelona & Valencia - 9 Days. This is a great March trip if you want to experience Barcelona without the huge crowds and experience the Fallas Festival in Valencia.
Madrid & the Andalusia Region - 14 Days. Start in Madrid, where you'll explore the capital's elegant streets and Prado Museum, before heading south to the Andalusia region to take advantage of its pleasant early spring weather.
More useful information
Spain in February
Spain in April
Best Time of Year to Visit Spain
How Many Days to Spend in Spain