Silly days for practical jokes are found all around the world. Perhaps the day started with the French, who once began the new year on the first of April. In 1564, when the new calendar began the year with January 1, some people resisted. They were considered April Fools.
Until 1564 April 1st was equal with the first day of Spring season, but due to the calendar changes in 1582 April 1st was moved ahead 10 days.
The Gregorian calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for the Earth to complete one orbit around the Sun.
The Julian calendar was switched over to the Gregorian starting in 1582, at which point the 10 day difference between the actual time of year and traditional time of year on which calendarical events occurred became intolerable. The switchover was bitterly opposed by much of the populace, who feared it was attempt by landlords to cheat then out of a week and a half's rent. However, when Pope Gregory XIII decreed that the day after October 4, 1582 would be October 15, 1582, the Catholic countries of France, Spain, Portugal, and Italy complied. Various Catholic German countries (Germany was not yet unified), Belgium, the Netherlands, and Switzerland followed suit within a year or two, and Hungary followed in 1587.
Because of the Pope's decree, the reform of the Julian calendar came to be known as the Gregorian calendar. However, the rest of Europe did not follow suit for more than a century.