Spring in Barcelona sees the locals out and about more, while visitors start streaming in, especially during and after Setmana Santa (Holy Week) and Easter. Temperatures are mild, though the humidity picks up, and while occasional rain through April and May tends to wash away the mugginess a bit, we usually don't get enough of the wet stuff to ruin a visit to the city. Lovely local festivals include Sant Jordi and L'Ou Com Balla, while one of the biggies is the Primavera Sound music festival, which brings in big international stars to take to the six stages over three days, as well as hundreds of thousands of music fans from near and far.
So many people choose to visit Barcelona in the warmest months that a common complaint in summer by tourists is that there are too many tourists. That perception could also be affected by the fact that locals tend to take their own holidays in the hottest season, especially in August, when you could find smaller businesses closed for the month. Most of the popular attractions, as well as city beaches, are packed, and you'll find long queues and fully booked accommodations. Flights and hotels are more expensive, and the humidity can be oppressive. Big draws are festivals such as Sant Joan, the Sónar music festival, Gay Pride, the city's huge performance arts Grec festival, and the wildly popular Gràcia street festival.
Early autumn in Barcelona is still warm, though cooling down enough to encourage more activities like long walks around the city or enjoying parks and beaches. Toward the end of autumn the weather cools off, and you might get some rainy days, but it's still considered quite warm for many who come from cooler climes. Generally there are fewer visitors than in summer, and it's a good time for sitting outside on terraces and watching the world go by. In September, Catalonia celebrates its national day on the 11th, and the 24th is La Mercè, the festival for the patron saint of Barcelona. The latter is celebrated across the city over four or five days with live music, traditional parades and dances, and loads more. Also in autumn, music and film fans are treated to the nearby Sitges Film Festival (featuring gore, horror, sci-fi and fantasy films) and the well-regarded In-Edit Beefeater Festival of music documentaries.
Though winter is the coldest time of the year, most of you reading this will laugh at the idea of what the locals in Barcelona find cold. There's still plenty of sunshine in winter, as well, and with temps between 5 and 15 degrees Celsius, on most days you can just grab a jacket and head out to enjoy the day. Winter in Barcelona can be really beautiful, with streets dressed up in lights toward the end of November and through early January, Christmas markets, fewer crowds, and even ice-skating rinks open for the season. Other than Christmas, New Year and Kings' Day, events in winter include the 080 Fashion week, the massive Mobile World Congress, and Carnival, which is still more popular here than Halloween for a costume party.