One of the best time of the year to visit Barcelona is during September, not just for the agreeable climate but also to celebrate the city's largest party, the "festival of festivals," the fiesta de la Mercé. It's an authentic artistic carnival featuring numerous activities like free concerts in all the squares, street art, children’s activities, contents, fireworks, and the popular Festa del Foc, where the streets fill with dragons moving the rhythms of drums and pyrotechnics illuminate the city in its path.
The party runs for various days and it's best to find out what's happening (since so much is happening) and what works best for your age and tastes. From rock, jazz, and folk concerts to plays, exhibits, beach parties, the Fiesta de la Mercé attracts visitors from all over the world. To come to la Mercé is to feel Barcelona more alive than ever, and that's really saying something for a city that practically never sleeps.