When to walk
The hiking calendar here is long, and the mistake most visitors make is assuming that July conditions look anything like April conditions. They do not.
Spring and autumn
The prime window: mild temperatures, green ground cover and the kind of low golden light that makes the coastal paths worth carrying a camera for.
Winter
The best time for Montseny and the Pyrenees if you want snow without alpine gear, and for Montserrat, where cold air clears the haze off the plain.
Summer
Walk Collserola or the coast early and finish by eleven, then save the exposed ridges for another trip. Heat and humidity on a July afternoon are not a joke.