The Diada de Sant Jordi commemorates the death of Saint George, patron saint of Catalonia as well as of England and many other countries, who apparently died on April 23rd 303.
Whilst the dragon-slaying legend is remembered what makes the day in Catalonia so special is that it is when lovers exchange books and roses.
In Barcelona, you'll find rose sellers on every street corner and La Rambla and Passeig de Gràcia are lined with tressel tables piled high with books.
Although Sant Jordi is not a Spanish public holiday and people have to work, as far as the Generalitat of Catalonia is concerned it is a Catalan National Day so you'll also see many senyeras - Catalan flags - and this why it is the Diada rather than the Dia de Sant Jordi.
Saint George's Day has been a Catalan holiday since 1456 and for some
reason has also been The Day of Lovers since around the same time.
However, the tradition of giving red roses, which apparently also symbolise the blood of the dragon, didn't begin until 1914 during the Mancommunitat or Catalan Commonwealth under President Enric Prat de la Riba.
The idea of having a Catalan Book Festival dates from 1927 and was originally celebrated on October 7th until two years later, during Barcelona's second Universal Exposition, the Barcelona booksellers decided to take advantage of the fact that the city was full of tourists and took their bookstalls out onto the streets.
The Ajuntament - the Barcelona City Council - decided that celebrating Saint George, a Lovers' Day and a Book Fair all on the same day was a good idea for a number of reasons.
Firstly, April 23rd was the day on which both William Shakespeare and Miguel de Cervantes died in 1616.
Coincidentally arguably the greatest Catalan prose writer, Josep Pla, died on the same day in 1981.
Secondly, since the Renaixença in the 1850s, the movement which gave rise to both the resurgence of Catalan as a literary language and later the artistic and architectural style known as Modernisme in which the symbol of Sant Jordi liberating Catalonia is seen with alarming frequency, Barcelona had reinstated a medieval literary competition known as the Jocs Florals in spring.
By moving the Jocs Florals to April 23rd, the Diada de Sant Jordi has become a day that is full of literary and historic significance so it is no surprise that Unesco designated it as International Day of the Book in 1995.
As far as I'm concerned La Diada de Sant Jordi is one of the most wonderful festivals and it's a particularly lovely time to be in Barcelona.
As you walk around the city, you get a strange high - I don't know whether it comes from smell of the roses or just the wonderfulness of the whole day.
I generally buy my rose the night before from Flores Navarro - the 24-hour flower shop on Carrer València - so I can wake my girlfriend up with it on the morning of La Diada de Sant Jordi, but I love the sight of seeing men carrying flowers throughout the whole day.
The book idea is so romantic and you'll see couples of all ages sitting on benches or outside cafés reading love poetry to each other.
Last year, I had already decided that I wanted a copy of George Orwell's magnificent Homage To Catalonia, which I lent to a friend many years ago and never got back.
It's not very romantic I admit but a Homage To Catalonia is definitely what the Diada de Sant Jordi deserves.
If you're just visiting Catalonia, La Diada is a day when you will really wish that you understood Catalan because there are free poetry and book readings in bookshops, libraries and theatres.
Along with El Dia de la Mercè, Sant Jordi is also the only day of the year when you can visit the inside of the Barcelona's two medieval government buildings - Palau de la Generalitat and Casa de la Ciutat, so most years I visit one or the other.
The sculpture of Sant Jordi in the patio of the Palau de la Generalitat is magnificent, by the way.
If you want a very special out of town visit, you should go south to the town of Montblanc in Tarragona Province.
This where, according to Joan Amades in the Costumari Català, Sant Jordi killed the dragon and a medieval pageant where the saint saves the princess is acted out every year.