In France, April Fool's Day is known as "Poisson d'Avril", translating to "April Fish", hence why the 2011 and 2012 events started around that date.
The Fishing chest has rewards from past Fishing events.
This table is only for when content was originally released. To find out all years something was available, view the page about the reward itself or view the event pages.
- April Fools' Day on Wikipedia