Christmas Is Creepy is written in the key of B♭ Minor.
According to the Theorytab database, it is the 11st most popular key among Minor keys and the 23rd most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (B♭ minor, E♭ minor, and F minor).
See the B♭ Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!