In English-speaking countries someone who's taken in by a prank on April 1 is known as an "April fool."
But in France, a prank on April 1 is known as a "poisson d'avril" -- an April fish.
Apparently Simeon is interested in studying such April fishes, because that's where he was found: looking into the fishpond.
For this he was made to wear an April fish for the day. That'll larn ya, silly monkey!
Simeon was also pleased to wear another hat today, this one more seasonally appropriate.
Happy April Fool's Day, and soon Happy Easter!