Head Covering needed?

no but needs to cover knees and shoulders

Not at all. Spain is no longer a Catholic country, as just 11% of population attend Mass and religious beliefs are quite low. A very liberal society where Church has lost most of its influence and young generations simply do not care about religion. The percentage of civil weddings is around 80% and when entering a church you may be requested to cover your shoulders (but in most churches nothing will happen if you don´t).

What about Mosques? (Seville, Cordoba, etc.)

Those haven’t been mosques for hundreds of years….

@Mike even the few us that are practicing Catholics wear shorts to Mass when it is warm. Like I did today.

@Emily, I think you mean mosques worth visiting. I go by one everyday on the way to work. It reminds me of those churches you find in a strip mall in the states.

Dlindstrom - I’m pretty sure that the OP doesn’t plan to visit a strip mall mosque in Granada… of course I am also aware that there are Muslim populations all over the world served by mosques. I live in Vienna and half of the staff I work with identity as Muslim.

So, do women have to have their knees/legs covered as well? Even in August?

Not at all, they can wear skirts inside the temples.

The cathedral in Barcelona had someone at the door back in 2015, seemingly turning away folks who were not properly dressed. I don't know what the rules were (or are), but there may be issues there--and Catalunya doesn't seem to be a particularly religious part of Spain.

Even if they dont have an armed multi-lingual bouncer at the door to explain what you should already know, you are supposed to dress modestly in any catholic church anywhere, anytime. Local people may have given up trying to keep that up, but it is disrespectful. Its not just a tourist attraction to many people, whether it is to you or not

I support Mikel's statements above, and yes, as Ann said, Catalonia (as well as Euskadi, Mikel's turf!) are probably the less religious areas in the whole of the Iberian peninsula.

Having said that, churches, much like (bear with me, let me use the term) any private enterprises, have their own rules on their premises. And yes, the Cathedral of Barcelona has a prominent sign at the main (and side) doors which is sometimes enforced -depending on who's at the door: no short shorts, no bikini, no bare arms (for women), etc. But nothing regarding head covering. Note that is not the case in the Basílica of La Sagrada Família, to my knowledge, there's no such sign there.