oh, this one... once you learn the lyrics it's hard not to sing along...
Ghouliday is the 40th track in the Undertale Soundtrack. It is a re-iteration of Jingle Bells, with all of the lyrics replaced with a "oo" sample. It is on the third CD found in Napstablook's house. The song continues playing after exiting their house. This (as well as Spooktune and Spookwave) can trigger an event where the protagonist encounters Aaron and Woshua, who are too scared to fight due to the spooky music. It consists of a single sample.
- Mad Mew Mew sings the lyrics to this song as "The 1000 Particle Subscriber Special" in Dog Shrine on Xbox versions of Undertale.