In the old days, new year was celebrated on the first day of every April. But as the world gradually switched to living by the Gregorian Calendar, new year was celebrated on the first of every January. Not everybody takes change well and some people continued to celebrate new year on April 1st -- these people (many of them French) were regarded as fools.

April Fools Day seems to have originated around 1752. Here is a calendar for September of 1752.

            September 1752
         Su Mo Tu We Th Fr Sa 
                1  2 14 15 16
         17 18 19 20 21 22 23
         24 25 26 27 28 29 30

Is this an April Fools joke?