A secret boss, So Sorry, can be encountered here. To do so, the game must be launched while the player's computer clock is set for October 10th, 8 P.M. If this condition is met, reading the sign causes So Sorry to run into the protagonist, initiating his encounter.
The Nintendo Switch version of the game has the sign removed the 8 P.M. part, allowing Nintendo Switch users to face So Sorry all day instead of 8 P.M. 
This room is inaccessible by normal means on the Genocide Route due to the second floor being blocked off by a force field. If file manipulation or other means are used to get into the Art Club Room, So Sorry still cannot be fought and the sign will read: "* (Art club is cancelled!)"
The room consists of 3 cacti and a sign in the middle that says the next art club meeting is on October 10th at 8 pm. A sign can be found in the empty space at the end of the hallway in the MTT Resort advertising the room.
- Today's the day you can go to the Art Club on Hotland 2F and have a special meeting. That's 8PM (your time). On the Switch version you can meet all day though!! - Toby Fox (@tobyfox) on Twitter, October 10, 2018.