World Book Day was fixed to honour two of the world's greatest writers, Miguel de Cervantes, the Spanish author (left in the picture), and William Shakespeare, the English dramatist (right in the picture). In 1995 UNESCO decided that the Day would be 23 April, because the death of each writer was given in historical records as "23 April 1616".

However, despite this attractive coincidence of the date, we can be sure that the two writers did not, in fact, die on the same day. The reason is that in the 1600s Spain and England were using different calendars. Spain had already switched to the Gregorian calendar, the one that is still used by most countries today; but England was still on the old Julian calendar (it switched to the Gregorian calendar in 1750). The calendars then differed by 10 days. So, Shakespeare's death, recorded (on the old calendar) as "23 April 1616", would have been recorded as "3 May 1616", had England already started using the present-day calendar.

There is one further complication. Some modern scholars believe that the date given for Cervantes's death may actually be the date of his burial and not the date when he expired, probably the day before. If so, he died on 22 April and was buried on 23 April.

So, the idea of choosing World Book Day to commemorate the deaths of the two writers was a neat one; but they really died 10 or 11 days apart.

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