Valentine's Day isn't too big in Catalonia, but that's because it has La Diada de Sant Jordi in April. It's like Valentine's Day in that men give women chocolate and flowers, but women give men books.
Once again, it goes back to marketing. In 1923 a bookseller wanted to honor Shakespeare and Spanish writer Miguel de Cervantes, who both died on April 23, 1616. Thus the book exchange was born.
Source: Flickr User Chocolate Geek