Q: From your experience, when would you recommend going to Barcelona when there are fewer crowds, but still pleasant weather?

A: For me personally I like late April or Mid September to the first week in October. These are good times to visit when Barcelona is a little quieter however it is generally still warm and sunny. However when we talk about the weather we can´t predict how it will be. All I can say is generally speaking these times of the year are quieter but still have fine weather.

Summary of the year-round weather in Barcelona
5 day weather forecast in Barcelona