all fools' day Meaning, Definition & Usage

  1. noun the first day of April which is celebrated by playing practical jokes
    April Fools'; April Fools' day.

WordNet

All" Fools' Day`
Definitions
  1. 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).

Webster 1913