World Book Day started in Spain in 1923 by a load of booksellers who wanted to mark the anniversary of the death of Migueal di Cervantes on the 23rd April, who was shot in the head from a book suppository by a man called Lee Herve Osvaldo.
Actually at that time it wasn't called World Book Day as it was only commemorated by people in Spain so I suspect they called it something like Spanish Book Day you get the general idea.
In Britain the day is celebrated on the 5th March as the 23rd April happens during school holidays and no teacher in their right mind is going to celebrate it when they could be in the pub. Personally I think it was moved to this day in Britain for reasons of pig-headedness but I can't prove that.