Catalonia's National Day, 11 September, commemorates Barcelona's capitulation to the Bourbon army in the 1714 War of the Spanish Succession, a bitter defeat that led to the repression of many Catalan institutions. It's lost some of its vigour but is still a day for national reaffirmation, with the Catalan flag flying on buses and balconies. There are marches throughout the city, the centre being the statue of Rafael Casanova (who directed the resistance) on the Ronda Sant Pere. Many make a pilgrimage to the monastery at Montserrat, Catalonia's spiritual heart.
If you're around for La Diada, you're sure to encounter the locals involved in marches and talks, but while you might not be able to spend the day shopping, you can join in a some of the activities and events going on throughout the day.