Washing the Lions
The April 2, 1698 edition of Dawks's News-Letter
(a London newspaper) reported that "Yesterday being the first of April, several persons were sent to the Tower Ditch to see the Lions washed." The joke was that there were no lions being washed at the Tower of London (although there were animals there). Therefore, the people had been sent on a fool's errand. This is the earliest known record of an April Fool's Day prank.
For well over a century after this, the prank of sending unsuspecting victims to see the "washing of the lions" at the Tower of London remained a favorite April Fool's Day joke. In the mid-nineteenth century, pranksters even printed up official-looking tickets that they distributed around London on April first, promising admittance to the (non-existent) annual lion-washing ceremony.