Winter
In Madrid, temperatures are seldom lower than freezing.   Good reasons to travel in the winter are:  low airfares (about ½ the high season fare), fewer tourist crowds, sale time beginning on Jan. 7, and special events like New Year’s Eve (eating 12 grapes as the clock strikes 12 at Puerta del Sol) as well as the Cabalgata, the Magi Kings parade on Jan. 5 in anticipation of their gifts on Jan 6.  Resort hotels which are packed in August are available at bargain rates in Jan. - early March especially in the Costa del Sol.  Weather there is in the 50’s (F)and 60’s. Low 20s Centigrade.

From the Tourist Office of Spain:  "Winter is a good season for the Mediterrean regions, particularly the Costa del Sol and the Balearic Islands.  The high mountain ranges offer excellent condition for winter sports.  The Canary Islands have an even and pleasant climate all year around."

Spring
For those who like pageantry, St. Joseph’s Day is celebrated most exuberantly with Las Fallas in Valencia, March 19.    This involves the construction and later burning of papier mache figures which satirize life.  Sevilla is most famous for its observation Semana Santa/Holy Week the week before Easter.  The weather is mild, but a rainy day is possible.

Tourist Office of Spain:  "Spring is an ideal time to visit the central regions of Castilla as well as Andalucia, the Mediterreanean Coast and the Balearic Islands.  The climate is generally very good, but expect occasional rain."

The weather in springtime Galiciacan be variable. Sometimes wet, sometimes sunny. The coast is usually damper and colder then inland Galicia. You should bring clothing to suit most conditions, but not shorts!! It is not really a time for sunbathing on the beaches of Galicia.

 

Summer
Summer is hot yet everyone in the world vacations in Spain.  In August the capital is deserted as the nationals themselves join the internationals at the beach.  Many smaller stores and restaurants are closed for summer vacations in August.  Cooler temperatures and smaller crowds recommend June as a month to go rather than July or August. 

Sophisticated travelers are discovering the beauty of northern Spain, like  Galicia, for their summer travel. At this time of year Galicia can get hot. Over the past three years average temps for August have reached as high as 40 degrees in the city of Ourense. It has been known to rain on occasions too. The coastal areas can be cooler than inland. The mountains like Ancares or O Courel are quite cool. Evening temps are perfect for sitting outside.

Tourist Office of Spain: "Summer is a good season to visit the northern regions.  The medieval pilgrimage route of Santiago de Compostela is ideal at this time of year." It should be noted that the mountains are cooler, but the route from Sarria to Santiago and the plains around Leon, can get extremely hot during August. The route is best walked during June and early August. There happens to be less travellers too!

Autumn/Fall
September is still warm while October brings cooler weather. The summer crowds have returned home.  Tourists can take advantage of the mostly pleasant weather and fewer crowds.  

Like the rest of Spain, Galiciacools down too. Weather in September is just great for sunny days without that intense heat of August. Rain occurs more often but without the rain you will never see a Greener Spain. Bring a jumper for those cooler evenings too. The colours of the tress, like elsewhere in autumn, are on the change. This only adds to the beauty of Galicia.

 

Tourist Office of Spain.  "Fall is perhaps the best season of the year to visit the entire country.  The climate is excellent with sunny days and blue skies." 

Weather is a very important consideration for planning your trip to Spain. This article is great, but there is some more very good advice on the subject at the website called climateandweather.com