Why do french people call it poisson d'Avril?

Answer 1

Same reason English people say something is "fishy".

From the "reliable" wikipedia:

If the exact origin of the use of fish remains unclear (perhaps ichthus Christian), it seems that many people found it difficult to adapt to the new calendar, others were not aware of change and continued to celebrate the new year on April 1, according to the old tradition. To mock them, some took the opportunity to give them false gifts and pranks. The gifts that were given in April were mostly food, as it is at the end of the Lenten fasting period during which the consumption of meat is forbidden among Christians, thus fish was the most common. When the jokes were developed, one of the most common pitfalls was offering fake fish.

Answer 2

....um.....where did you here this?......i have never heard french people dot this before......