Here is one theory as to how we came to celebrate the first of April with foolishness!
Everyone has a different answer, but this one sounds the best to me.
The entire world now uses what is called the Gregorian calendar. But originally,
a different calendar was used to tell time. It was called the Julian calendar.
It was created either by Julius Caesar or under his rein. At some point in time
many years after the fall of the Roman Empire, the monks found that the calendar
was off by so many days so it had to be revised. Now according to legend, the
Julian calendar (or perhaps another version of the calendar) celebrated the
New Year on April first, but the revised calendar made January the beginning
of the New Year. So if you were still celebrating the New Year on the first
of April, you were considered to be an April fool!