La Diada de Sant Jordi, also known as el dia de la rosa (The Day of the Rose) or el dia del llibre (The Day of the Book) is a Catalan holiday held on 23 April, with similarities to Valentine’s Day and some unique twists that reflect the antiquity of the celebrations. The main event is the exchange of gifts between sweethearts, loved ones and respected ones. Historically, men gave women roses, and women gave men a book to celebrate the occasion—”a rose for love and a book forever.” In modern times, the mutual exchange of books is also customary. Roses have been associated with this day since medieval times, but the giving of books is a more recent tradition originating in 1923, when a bookseller started to promote the holiday as a way to commemorate the nearly simultaneous deaths of Miguel Cervantes and William Shakespeare on 23 April 1616.
In Barcelona’s most visited street, La Rambla, and all over Catalonia, thousands of stands of roses and makeshift bookstalls are hastily set up for the occasion. By the end of the day, some four million roses and 800,000 books will have been purchased. Most women will carry a rose in hand, and half of the total yearly book sales in Catalonia take place on this occasion.
Source (as much as it pains the academic in me): Wikipedia