According to the Bigastro web site, on the 23 of April, 1616 three great writers of universal literature; Miguel de Cervantes, William Shakespeare and Garcilaso de la Vega passed away. That is apparently why UNESCO chose 23rd April to commemorate books.
The Council of Culture and Library in Bigastro have organized, for the 21st and 22nd April, their second "International Library Day" to be held in the Parque Municipal Huerto del Cura.
Actually Miguel de Cervantes and William Shakespeare did both die on the 23rd April 1616 but apparently Garcilaso de la Vega passed away on the 14th October 1536 so I'm not quite sure why he is included.
Never mind I imagine there will be a lot of interesting books to peruse.