Why Halloween? Here's my guess
It is no secret that Tatsuki is a huge cinephile, and I believe Cosmo is no exception to this.
Take Tim Burton's The Nightmare Before Christmas, for instance.
The film opens by a musical, introducing its setting, which directly shows how Tatsuki drew inspiration for this specific sequence.
The influence is especially apparent during the iconic "This is Halloween!(x2) Halloween(x4)!"
In the movie, Jack Skellington—the Pumpkin King of Halloween Town—wants to invite Santa Claus to his realm, respectfully referring to him as "Sandy Claws."
This particular detail made me realize why the Santa dolls in the manga have hands that turn into blades.
Furthermore, the way Santa is neutralized by Cosmo—being abducted into her library of minds and left entirely broken,
capable of chanting nothing but "Halloween"—strikingly echoes Santa's kidnapping in Burton's film.