Sant Jordi, or St. George, is the patron saint of Catalonia and the Diada de Sant Jordi, St George’s Day, is a festive occasion that, over the years, has become a celebration of Catalan culture. This is why many balconies in the city are decorated with the senyera, the Catalan flag, as they are on other key national dates. In fact, it is a celebration of culture and love, represented by books and roses. The main meeting point in Barcelona is the Rambla, which is packed from end to end with book and flower stalls.
On Sant Jordi’s day it is normal to find books in every language that talk about Catalonia but there are lots of stalls that specialise in books in Catalan, especiallly as this day is seen as a time for promoting and defending the Catalan language and culture.
More here: http://lameva.barcelona.cat/culturapopular/en/festivals-and-traditions/sant-jordi