CDP theme day this April 1st... April fool's day. More fun here.
But, why a fish??? It's a French thing.
In 1508, French poet Eloy d'Amerval referred to a poisson d'avril (April fool, literally "April's fish"), possibly the first reference to the celebration in France.Some writers suggest that April Fools' originated because in the Middle Ages, New Year's Day was celebrated on March 25 in most European towns, with a holiday that in some areas of France, specifically, ended on April 1,and those who celebrated New Year's Eve on January 1 made fun of those who celebrated on other dates by the invention of April Fools' Day. The use of January 1 as New Year's Day became common in France only in the mid-16th century, and that date was not adopted officially until 1564, by the Edict of Roussillon.