Christmas is a lovely time for songwriters too. Give yourself the gift of an expanded toolbox and the joy of knowing more than Four Pillars of Western Civilization (root, IV, V and the VI or relative minor). Some of these lovely Christmas chestnuts from the Great Songbook feature a wonderful opportunity to understand substituting chords to rotate to a different apparent tone center.
An example; take key of C. Its companions are F (the IV) and G (the V). Swap that F for its relative minor Dm, and now you have a II-V-I of Dm - G - C. Next go to another chord and let IT become a new IIm for a moment, like G - make it Gm. Now you have a new II-V-I, Gm - C- F, and it sounds like you changed the key to F. A similar trick can buy you another three chord progression to get home, or some other interesting place along the way.
Some of these songs have fascinating and delightful changes like this, they're fun as heck to sing, and then you've got some new ideas to take back to your own songwriting desk. Have fun learning a Christmas tune or two this season, open up the hood and have a look at the underlying theory, and you'll have yourself a real gift. Merry Christmas to you, wherever your songwriting desk may be