The origin of this celebration has no official version, however, the best known dates back to 1565, when King Charles IX had the dates of the New Year's celebration changed after adapting to the Gregorian calendar. 

Before King Charles' request, the New Year was celebrated on April 1st, after the beginning of spring, but with the change to the Gregorian calendar, the beginning of the year was moved to January 1st, as it is still celebrated today. 

Due to the abrupt change of date, many people were slow to learn of this fact and continued to celebrate April 1st as the beginning of the new year, bringing with it mockery from those who had been aware of the change in the date. 

As a joke to the people who celebrated April 1st, the other citizens sent them a piece of paper in the shape of a fish to stick on their back and this habit of joking with each other continued for decades in France, Great Britain and Scotland, who in turn transmitted it to the American colonies.

Today, this day is still celebrated with jokes between people, some in person and others through social media, so you have to be very careful with what you perceive on this date.