Today, Sant Jordi is celebrated on 23rd April and it is known as the day of the book and the rose. It celebrates Catalonia's patron saint and coincides with Sant Jordi's birthday. According to the legend, Sant Jordi killed a dragon which was frightening the people in his town and so the dead dragon's blood became a rose which he gave to a princess. The tradition states that men should give a rose and women should give a book to their loved one on the 23rd April.
Each year the streets in Barcelona, especially the Rambla, are filled with stalls selling books and roses. Famous authors also sign copies of their books in the street or in book shops. In 1995, UNESCO declared the 23rd April the International Day of the Book.
I was in Barcelona on Sant Jordi and I think it is a lovely tradition. You can pick up some great books on the Rambla.