On April Fools’ Day in France children make and decorate paper fish and then tape them to people’s backs. When their “victim” finds the paper fish on his/her back the person who put it there calls out “Poisson d’Avril” (“April Fish”).
If you want to play this joke on unsuspecting friends and family members but don’t have time to make your own fish here is a link for some you can print out: http://jlongok.com/printable-fish-pictures/
I have inserted a video below that explains (in French) the origin of the holiday.