A well accepted origin for April Fool's day is that when the western world adopted the Gregorian Calendar which switched the first day of the year from (about) late March - early April to Jan 1st that not everyone knew/accepted the new system which led to traditionalists celebrating the new year on April 1st vs. the newly accepted January 1st. The people who used the new dating system would make fun of the traditionalists for still using the April new year system and the tradition of making fun of evolved into pranking others eventually.
Of course, some have doubts in this because this was a well established tradition that was used in England even before England adopted the Gregorian Calendar.
Random trivia on the subject, may have gotten a few things wrong. Just putting it out there for those who are interested 
XD
