Why its April Fool's Day?

Six million Britons went to bed on September 2, 1752, and woke up on September 14. The reason? The Calendar (New Style) Act of 1750.

The mystery begins in 1582 when  Pope Gregory XIII was reigning as leader of the Roman Catholic church. He discovered that the Julian calendar that the church used at the time measured a year as 365 days and 6 hours long.

Actually the average length of a year is 365 days, 5 hours and 49 minutes. 

Over 1300 years the error became very significant. So he released a Papal bull (a declaration from the leader of the Catholic Church) —decreeing that every country would have to shift to a new calendar and skip some days.

Britain being mainly a Protestant nation cared little for the Pope's wishes. Soon many Europeans and many parts of the world changed from the Julian Calendar to the Gregorian Calendar. Eventually 168 years later Britain too decided to shift.

So the King ordered that 11 days be wiped off the calendar in Sept 1752. The workers worked for only 19 days but were paid for 30 days. Some historians claim that is how the concept of 'paid leave' was born. 

In the Roman Julian calendar the year began on 01 April, whereas the Gregorian calendar began on 01 January. Many Britons refused to change tradition and continued to celebrate 01 April as New year's day.

When royal orders failed to stop the celebrations on 01 April, the King decreed that all those who celebrate New Year's day on 01 April wold be called 'fools'.

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Ref:  Why the English Calendars Skipped 11 Days in 1752?