Source : Webster's Revised Unabridged Dictionary (1913)

All Fools' Day \All" Fools' Day`\
   The first day of April, a day on which sportive impositions
   are practiced.

         The first of April, some do say, Is set apart for All
         Fools' Day.                              --Poor Robin's
                                                  Almanack
                                                  (1760).

Source : WordNet®

All Fools' day
     n : April 1; celebrated by playing of practical jokes [syn: {April
         Fools'}, {April Fools' day}]