If you are planning on a busy sightseeing holiday, then the best time to visit Spain is in the spring (late March to May) or autumn (September to early November), when the temperatures are neither too hot nor too cold.
In the winter (generally November to March), the mountainous regions in the north and in particular the central plateaux become very cold. Other areas of the country, even if milder, can see more rainfall, particularly the regions of Galicia, Asturias, Cantabria and the Basque Country which are influenced by the Atlantic Ocean and have a maritime climate.
In the summer (May to September), the central plateaux of Spain go to the other extreme and experience very hot temperatures. The far south also becomes very hot with an almost African climate. To the north, summer temperatures are more comfortable and there may still be the chance of rain in the regions along the Atlantic coast, which is why these areas remain so wonderfully green in comparison to the rest of Spain at this time of year.