The Basque has two differented weather zones: one by the coast, Maritime climate, and the other in the south where the Mediterrenean climate prevails. There is a transition area in between in the Llanada de Vitoria where the Maritime climate mixes with the Mediterranean.

Temperatures are not extreme. In the coast, temperatures are an average of 19 ºC  in Summer and 9 ºC in Winter. The highest temperatures can hit the mid 30's ºC  when Foehn wind blows and when the Siberian anticiclon settles in, temperatures can go as low as 0 ºC. Llanada de Vitoria is the coldest area with average temperatures of 5 ºC in Winter and 20ºC in Summer.

The region is affected by the polar front that brings rain and polar maritime air currents, the Azores anticiclon that brings maritime tropical air currents and creates the dry and sunny waeter, the Siberian anticiclon that brings polar continental cold air currents creating a cold and dry season.  

In Basque, it rains a lot year round and almost 65% of the days are cloudy. It is the cloudest region of Spain. You will find that in Autumn and the beginning of Winter it is when it rains the most and June and July when it rains the less, but it still rains. The Atlantic zone registers the more contant rain including the typical "txirimin" or fine rain.  

If Summer may be the best season to go in terms of climate, July and August may be crazy months to go to Basque. Spanish and European holidaymakers crowd highways, beaches and attractions. Perhaps this is more of a problem in San Sebastian than in Bilbao as San Sebastian is a truly tourist city.