This is an old French postcard for April 1. The words at the bottom "Ils diront celle qui vous aime" translate as "They say one who loves you."
Fish are featured on many early 20th-century French April Fools' Day postcards. In France and some other countries, the April Fool's Day tradition is known as "April fish" (poisson d'avril in French). This involves attempting to attach a paper fish to the victim's back without being noticed.