Ever wondered why Sain Valentine is celebrated on the 23rd of July in Barcelona? Here's the story behind the tradition of the Diada de Sant Jordi, also the national day of Catalonia! 
Sant Jordi Day
Sant Jordi (or St. George in English) is considered to be the patron saint of Catalonia –and of some other countries in the world-. Sant Jordi's Day is the Catalan equivalent to St. Valentine's Day, when roses and books are given away between lovers as a symbol of their love. It is also the celebration of Catalonia's "national" day.
Origin
According to the legend, a long, long time ago a terrible dragon used to harass the people of the small village of Montblanc, in Catalonia. 
The dragon would devour every animal in the village, and wreck havoc amongst the locals. In an attempt to calm the dragon's anger, the people of Montblanc decided to offer him a person in sacrifice each day. That person would be chosen by drawing of lots, until one day it was the king's daughter who luck wanted to be the one offered to the dragon. When she was just about to be swallowed up by the dragon, a brave knight -Sant Jordi- appeared and thrust his lance into the dragon, slaying it to death. Out of the blood that was spilled, there grew a red rose bush.
Reality is that Jordi (or George, in English) was a 4th century Roman soldier, made a martyr by the Catholic church when he was killed for his religious beliefs. In any case, the saint was chosen as the patron saint of Catalonia, and the tradition of giving away roses was popularized amongst sweethearts in Catalonia. Also, during the Renaixença, the XIX century cultural and social movement that wanted to restore the symbols of the Catalan identity, Sant Jordi was taken as the movement's iconic figure. The tradition is that men give a long stemmed rose to the ladies, while the ladies give away a book to the men. This last part was originated in 1923, when a very strategic and intelligent bookseller had the idea of commemorating the date of death of two of the main literary figures in History: the English author William Shakespeare and the Spanish genius Miguel de Cervantes, both deceased on April 23rd, 1616. This is how he decided that it would be a great business move to use the Diada de Sant Jordi to celebrate the World Book Day, and make the ladies gift a book to men.
Events
The streets of Barcelona and Catalonia are on the Diada covered in senyeras (the Catalan flag), and are also present as decoration on the roses and rose stalls, booksellers' stalls, facades and windows; one can see them basically everywhere in town. The typical colour for the roses is red, according to the legend, but nowadays they can be found in many different colours. There is also an interesting initiative from the local council that receives the name "Roses contra l'olbit", which gives roses to the elderly and lonely, granting that every woman in Barcelona has her rose on that day. Finally, you should keep in mind that almost every celebration or festivity (in Spain and Catalonia) is accompanied by a cake or a special pastry. So remember to visit a bakery in town to try the typical pastries specially cooked for this event.
So, summing up, the Diada de Sant Jordi is a day for love in all its forms: romantic love, family love, and solidary love, so you can also consider contributing to the many charity oriented stalls that sell roses and book down Las Ramblas, for example. And always keep this in mind: the love you give is the love you get!