The Poppy Collaboration Event released on September 24th, 2021, and ended on September 27th, 2021.
It was an event similar to the Twenty One Pilots event, where it's merely just a collaboration with another group and isn't related to CoS at all.
The event required players to find 9 musical notes, each playing a song made by Poppy who is an "American singer, songwriter, musician, and YouTuber". If a player found all 9 notes, they unlocked a color palette for Cottol.