In America, we say “April fool’s!”
In Italy, we say “Pesce d’aprile” or April Fish.
I’m told the name comes from a common joke that involves sticking a drawing of a pesciolino (little fish) onto the back of an unsuspecting victim. Then, everyone else asks if anyone has seen “April’s fish” and makes jokes about that person – when, of course, the victim doesn’t know it’s them.
L’hai visto? (Have you seen him/her?)
Il pesce d’Aprile! (The April fish!)