all fools day

from The Collaborative International Dictionary of English v.0.48

All Fools' Day \All" Fools' Day`\
   The first day of April, a day on which sportive impositions
   are practiced.
   [1913 Webster]

         The first of April, some do say,
         Is set apart for All Fools' Day.         --Poor Robin's
                                                  Almanack
                                                  (1760).
   [1913 Webster]