There’s a long tradition in many countries to celebrate April Fool’s day on April 1st. That’s when people play practical jokes and hoaxes on each other .
Interestingly in France (and also Italy and Belgium) this tradition is closely related to fish and the day is called “April Fish” or in french Poisson d’Avril. Traditionally people stick paper fish on the back of each other and shout “April Fish!” . The commonly believed reason why the do so is historically based in France after the adoption of the reform calendar by Charles IX in 1564. When New Year’s Day was changed to January 1st people who still celebrated New Year’s Day on April 1st were the ‘butt’ of many jokes. They were called “Poisson d’Avril” (April Fish) because at that time of year the sun was in the zodiac sign of Pisces, the fish. 
Just to stick to that relation of April 1st and fish:
Do you know the fur-bearing trout?
“The Fur-Bearing Trout is a species of fish that possesses a thick coat of fur to keep itself warm in the cold waters where it lives. These furry fish are primarily found in the northern regions of North America, but particularly in Canada, Colorado, Wyoming, and Montana. The species is also sometimes referred to as the Beaver Trout, or (incorrectly) as the Sabled Salmon.
A number of theories have arisen to explain this creature’s luxuriant coat:
- Some say that the creature evolved its thick coat to protect itself from the extreme cold of northern waters.
- According to another lesser-known theory, this species of trout owes its fur to four jugs of hair tonic that were accidentally spilled into the Arkansas River (in Colorado) sometime during the 1870s.
A few researchers suggest that as the weather grows warmer during the spring the fur-bearing trout sheds its fur, only to regrow its coat as winter returns. This may help explain why trout with full coats of fur are so seldom encountered.
According to legend, the fur-bearing trout was first encountered by Europeans when Scottish settlers emigrated to Canada during the seventeenth century. One of them wrote home remarking about the abundance of “furried animals and fish” in the new land. Asked to provide more information about the furried fish, the settler duly sent home a specimen. If true, this legend would pretty much discredit the Colorado hair-tonic theory of the creature’s origin.
Fur-Bearing trouts mounted as trophies can be found hanging on walls throughout the Great Lakes region of North America.”