World Book Day is celebrated on this day because it was once thought that both Miguel Cervantes and William Shakespeare died on 23 April 1616.
However, while it was beleived that they both died on the same date, it was known that this didn’t actually take place on the same day.
To explain, Spain had already at that time switched to the Gregorian calendar, while England still used the old Julian calendar (still in use today by Orthodox Christians).
Thus Shakespeare actually died on 23 April 1616 according to the old calendar, which is 3 May 1616 according to the calendar we use today. All in all, Shakespeare did not die on this day in the true sense of the word.
To make matters worse it has recently become known that not even Miguel Cervantes died on this day. In fact, he was buried on this day in 1616, but actually died one day earlier, i.e. on 22 April, in Madrid.
Older encyclopedias and biographies cite 23 April 1616 as his date of death. Newer encyclopedias are gradually adopting 22 April for the death of Cervantes.
Therefore, although neither Shakespeare nor Cervantes actually died on this day, up until recently it was erroneously accepted that they did. That presumption of correlation led UNESCO to declare this day the World Book and Copyright Day in 1995.
Public readings of Cervantes’ The Ingenious Gentleman Don Quixote of La Mancha, rated by many experts as one of the first and best novels in world history, are organized on this day throughout the world.