It was a mix of a few past April Fools Day parties from Club Penguin.
The dimensions from the 2011 party appeared, however the additional ones introduced in 2012 did not.
It was confirmed by Codey in-game and then it was officially announced on the official CPR Twitter account.
It started the day before the original Club Penguin closed.
A glitch occurred on the last day of the party where players could go back to the Mountain Expedition rooms. This glitch only worked for people who did not clear their cache on April 11th, 2017, or when the Easter Egg Hunt started. Not much about this glitch is known since it happened only on one day.
It was during this party Club Penguin Rewritten saw a massive influx of penguins, ballooning to nearly 100,000 registered penguins following the original game's shutdown.