What time of year to visit northern Spain?

Living here all I can say the weather can and will be variable.
Not only the weather systems will be unpredictable but your location will also will have an affect on what you find.
The normally a rule of thumb is; the more in land you are the dryer the weather is.
You should not find any snow at either time of year.

September, Yes; June, No!
Given your situation & proposed itinerary I would Opt for May. With your apparent flexability, consider flying into Vigo (Pontevedra). After spending a day or two in Pontevedra, you can take a TRD (Tren Rapido Diesel) Vigo to Santiago in about 1.5 hours

In Vigo, there's a Bagpipe (Gaita Gallega) festival each May. Carlos Nuñez won it about 10-years ago before he joined The Chieftains (Irish Folk Group). He's from the area and later played Ulliean Pipes with the Chieftains at Dublin in Vigo, Vigo's principal Irish NightClub.

You'll probably encounter some rain in Galicia - it's very much like Ireland: Green; great Potatos; Bagpipe Music; and a restored Celtic Village from 300 AD (Monte Santa Tecla). It's a National Historic Site

We´ve had a fantastic weather in October in Bilbao this year, as well as September. But, as ribeirasacra says, weather in northern Spain is quite unpredictable. In any case, it may rain, it may not, it depends on the weather...

Just to add a personal anecdote. This past March we went to Santiago and spent 4 days there. We had gorgeous weather, we could sit outside during the day with a light sweater (and sometimes none was needed) and then temps went down in the evening. Also, no rain wthatsoever , except some drizzle one night. All in all , we had better weather than we previously had in other visits during the summer. You never know!

Thanks to all for your informed opinions. When reading about the Picos de Europa it was suggested that “visitors who are serious about hiking prefer September because of dryer weather”. This is when I started to consider moving my trip from May to September. I don’t mind a little rain; after all I live in Miami. I just don’t want days of overcast, gloomy weather. The high sixties or low seventies would be perfect temperatures but not higher. I live in the heat and long to escape.

If you like to swim I think the water might be warmer in October. The beaches of Northern Spain are fantastic.

We were in Santander at the end of Sept. , beginning of Oct last year. The weather was mostly sunny and warm, but not too many people were swimming. Toward the end of our stay we had some heavy ( but short) rain!

This year in mid-July, it was cold and rainy in Santander..

but as everybody says, weather is never predictable at the Atlantic coast.

Northern Spain has fantastic beaches, especially around Santander (Liencres is spectacular, and Somo, just right across the bay from Santander, is also most beautiful). If you want to go swimming, July to September will be the season.

If you do not swim, May or October are good as well for sightseeing and hiking.

And do not forget the fabulous restaurants...

Do you need any more tips about the region?

You can swim any time of the year in the hot springs dotted around southern Galicia.
Just to prove the unpredictable bit Galicia in July this year was fabulous.

Greetings fellow Fodorites. I have until November 23 to decide on whether or not to buy my tickets for May. Of course I (and must you!) love to travel and prefer sooner rather than later.

The swimming issue is really mute for us. We live blocks from a beautiful beach so that activity has little novelty for us.

Anything older than the 1930’s and terrain not pancake flat is what we are after! Plenty of that in northern Spain. It seems that the general consensus is to stick with our travel dates in May. I just don’t want it too dark and rainy.

They say one out of every four days is sunny on the Northern coast.

that much?...mmm, I´d say "not rainy" instead of sunny.

Well, to be honest, it´s quite impossible to predict the weather over here. It´s one of our main topics of conversation, and we always carry an umbrella or an overcoat just in case...

In Galicia the locals always carry an umbrella either to shelter one from the sun or from the rain!