I wondered why all these vintage post cards featured fish.
Fish are funny and foolish when they are applied to ordinary situations.
So I checked Wiki- Now I know about April Fish!
April Fools' Day is celebrated in many countries on April 1 every year. Sometimes referred to as All Fools' Day, April 1 is not a national holiday, but is widely recognized and celebrated as a day when people play practical jokes andhoaxes on each other.
In Italy, France and Belgium, children and adults traditionally tack paper fishes on each other's back as a trick and shout "April fish!" in their local languages (pesce d'aprile!, poisson d'avril! and aprilvis! in Italian, French and Flemish, respectively). Such fish feature prominently on many French late 19th to early 20th century April Fools' Day postcards