Correfocs, or fire-runners, are one of my favorite elements of Catalan culture and local festivities in Barcelona. Dressed as demons and dragons, locals of all ages (kids, too) don fire-resistant fabrics and dance and skip around town spewing fire at onlookers to the beat of marching drummers.
While one of the best times to see the fire-runners is during the city-wide festival, La Mercé, there's also occasion at yearly festivals in Barcelona neighborhoods and nearby communities like Esplugues de Llobregat, Badalona and St. Joan Despí. There's also Barcelona's winter festival, La Laia. Whatever you do, dress accordingly--you don't want to go up in flames when the sparks start flying and bring your camera--you'll want pictures.