Overall, spring, early summer and autumn are ideal times for a Spanish trip - though the weather varies enormously from region to region. The high central plains suffer from fierce extremes, stiflingly hot in summer, bitterly cold and swept by freezing winds in winter. The Atlantic coast, in contrast, has a tendency to damp and mist, and a relatively brief, humid summer. The Mediterranean south is warm virtually all year round, and in parts of Andalucía positively subtropical, warm enough to wear a T-shirt by day even in the winter months. 

In high summer the other factor worth considering is tourism itself. Spain plays host to some thirty million tourists a year - almost one for every resident - and all the main beach and mountain resorts are packed in July and August, as are the major sights. August, Spain's how holiday month, sees the coast as its most crowded and the cities, by contrast, pretty sleepy.

Average temperatures
Jan  March  May  July  Sept  Nov
Madrid (°C)  9  15  21  31  25  13
Castile (°F)  49  59  70  88  77  56
Málaga (°C)  17  19  23  29  29  20
Costa del Sol (°F)  63  67  74  84  84  68
Sevilla (°C)  15  21  26  35  32  20
Inland Andalucía (°F)  59  70  78  95  90  68
Pontevedra (°C)  14  16  20  25  24  16
Galicia (°F)  58  61  68  77  75  61
Santander (°C)  12  15  17  22  21  15
Cantabrian coast (°F)  54  59  63  72  70  59
Barcelona (°C)  13  16  21  28  25  16
Catalunya (°F)  56  61  70  83  77  61
Cap Bagur (°C)  14  16  20  27  25  16
Costa Bravo (°F)  58  61  68  80  77  61
Alicante (°C)  16  20  26  32  30  21
Costa Blanca (°F)  61  68  78  90  86  70
Mallorca (°C)  14  17  22  29  27  18
Balearic Islands (°F)  58  63  72  84  80  65

Note that these are all average temperatures - and whilst Sevilla, the hottest city in Spain, can soar into the nineties at midday in summer, it is a fairly comfortable 23-27°C (75-80°F) through much of the morning and late afternoon. Equally, bear in mind that temperatures in the north, in Galicia for example, can approach freezing point at night in winter, whilst mountainous regions can get extremely cold at any time of year.