Mid-April through June, September, and October are the best times to enjoy the temperate climate and both the coastal and upland landscapes of this wet and grassy corner of Spain—though any time of year except August, when Europeans are on vacation, is nearly as good.
Pamplona in July is bedlam, though for hard-core party animals it's heaven.
The Basque Country is rainy in winter, but the wet Atlantic weather is always invigorating and, as if anyone needed it in this culinary paradise, appetite-enhancing. Much of the classically powerful Basque cuisine evolved with the northern maritime climate in mind.
The September film festival in San Sebastián coincides with the spectacular whaleboat regattas, while the beaches are still ideal and mostly uncrowded.
When you're looking for a place to stay, note that the largely industrial and well-to-do north is an expensive part of Spain, which is reflected in room rates. San Sebastián is particularly pricey, and Pamplona rates triple during San Fermín in July. Reserve ahead for nearly everywhere in summer, especially Bilbao, where the Guggenheim is filling hotels.