As I’ve posted in the past whenever a party on Club Penguin is over (or when Rockhopper leaves) you can still access a few of the rooms, in this case it’s for the Halloween Party! You can still access the Haunted Mansion, Haunted Mansion Entrance, and Secret Lab.
To do this, use the following link to log into Club Penguin. From there your penguin will start off in the Mansion and you can waddle to the other rooms. You cannot access the Dark Chamber.
Special thanks to Echo006 for reminding me about this!