April Fools' Day is celebrated on the first day of April.
The 2015 April Fools Event was a scavenger-hunt-based event. The 2015 April Fools event used a "Paper Fish" theme. It was a reference to the French and Italian April Fish pranks traditionally done on the holiday. 
Each time a page loaded, there was a chance that a darkened version would appear in place of the normal page. A spotlight surrounding the cursor would also appear. The darkened page would function normally, actively responding to all actions done on the page, e.g. Interactions on a person's User Page. Using the spotlight around the cursor, a user would be able to search for a Pokémon sprite placed randomly around the page.
The sprites that appeared varied between "Paper Fish" and normal Water-type Pokémon sprites. There were 7 unique variations of Paper Fish, each representing a Water-type Pokémon wrapped in paper. When a user found and clicked on a Paper Fish, a "Fish!" popup would appear displaying the number of Paper Fish that user had found. Normal Water-type Pokémon that appeared did not count toward the event goal, and were used as annoying Red Herrings (jokes). 
Custom SpriteOrigami Goldeen Custom Sprite at the conclusion of the event. The chance of receiving a prize from the event rose with each additional Paper Fish found. Finding 5 of each Paper Fish variation would guarantee a Custom Sprite. 
Custom Sprite Goldeen was released on April 8th. Users who collected at least one of each type of Paper Fish were sent a Private Message with a special Cheat Code that would allow them to collect their prize. The probability of the Cheat Code yielding a CS Goldeen was determined by the number of total Paper Fish collected.
2014Summary Page, the owner's name would show up as CATS, one of the site's Non-Playable Characters.
The event is a reference to Zero Wing, a video game which has become infamous on the internet for its extremely bad translation. CATS' Trainer Card note, farm name, and Pokémon descriptions are all pieces of dialogue from the game's intro. His user description has a music player, which plays the game's intro theme.
Following announcement was made after April Fools.
In April 1st... war was beginning. What happen? Somebody set up us the bomb. We get signal... main screen turn on... it's them! "How are you gentlemen!! All your Pokémon are belong to us. "You are on the way to destruction. You have no chance to survive make your time. "Ha ha ha ha!!" What they say!! You know what you doing... For great justice, take off every Espurr. Move 'purr. Move 'purr. Move 'purr. Move 'purr. You know what you doing, take off every 'purr. CONGRATULATION!! April 2nd All bases of CATS Were destroyed. It seems to be peaceful. but it is incorrect. CATS is still alive ... Niet seems to have a tendancy to save the bad guys. First False Prophet and now CATS. You sure he's not secretly evil? Anyway... PFQ-01 must fight against CATS again. And down with them completely! Good luck.