April Fools' Day
noun
or
April Fool's Day
or
April Fool's Day
Britannica Dictionary definition of APRIL FOOLS' DAY
[singular]
:
April 1 celebrated as a day on which people play tricks on each other
— called also
All Fools' Day
◊ A trick that is played on April Fools' Day is often called an April Fools' trick or April Fools' joke. A person who is tricked on April Fools' Day is sometimes called an April fool.