April fools day was a result of the calendar adjustment made from switching from the Roman (Julian) calendar to the Gregorian calendar, causing a shift and just jumping from the current Julian date to April 1st when they converted to the Gregorian calender in the late 1500's ......... Surprisingly, it took almost 3 centuries to get all the European countries to convert to the common calendar we all take for granted. All that is left is the April fools day