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April 23rd is World Book Day because, in theory, Cervantes and Shakespeare died on the same date in 1616. The real story is twistier.
Cervantes died in Madrid on April 22nd and was buried on the 23rd, which is the date that went on record. Shakespeare died on April 23rd... of the Julian calendar, which England still used while Spain had already switched to the Gregorian one. Converted to our calendar, the Englishman actually died on May 3rd.
So about eleven days separate the two deaths, whatever the paperwork says. And there's a third guest at this coincidence: Inca Garcilaso de la Vega, the great chronicler of Peru, is also recorded as dying on those very dates.
World Book Day celebrates, deep down, a calendar error. A beautiful one, admittedly.