In 1563 King Charles IX reformed the French calendar by moving the start of the year from Easter Day to January 1. His edict was passed into law by the French Parliament on Dec. 22, 1564. This aligned legal convention with what had long been the popular custom of celebrating the start of the year on January 1.
Later, in 1582, Pope Gregory issued a papal bull decreeing sweeping calendar reform, which included moving the start of the year to January 1, as well as creating a leap-year system and eliminating ten days from the month of October 1582 in order to correct the drift of the calendar. The Pope had no formal power to make governments accept this reform, but he urged Christian nations to do so. France immediately accepted the reform, since it had already instituted part of the reform (changing the start of the year) in 1564.
This sixteenth-century calendar reform is frequently cited as the origin of the custom of April Foolery. Supposedly the people who failed to realize the start of the year had been changed had pranks played on them on April 1st.
There are a number of problems with this theory. First, the start of the year was changed from Easter day, not April 1st. Second, January 1st had, since Roman times, been the traditional start of the year anyway. Easter Day had been used as the start of the year primarily for legal and administrative purposes (in an attempt by medieval rulers to christianize the calendar).
The calendar-change hypothesis is more plausible if applied to Britain, where March 25 (the date of the christian Feast of Annunciation, aka Lady Day) was New Year's Day, followed by a week of festivities culminating on April 1. However, Britain only changed the start of its calendar year to January 1 in 1752, by which time April Fool's Day was already a well-established tradition.
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