The April Fools' Day tradition is widely celebrated on April 1st. across much of Europe and the Americas.
The origins of the tradition appear to have have been lost.
Several conflicting theories have emerged - such as the idea that it's a continuation of the ancient Roman tradition of Hilaria , which were comedic festivals to celebrate the vernal equinox (19th - 21st March). It may have moved to April 1st as a result of changes to the calendar.
Others point to the works of Chaucer (1392) which appear to mention April 1st as a comedic date (though some scholars claim the reference was simply a printing error).
A third theory centres around the tradition of "April Fish Day" (poisson d'avril in French).
Further reading Time Magazine