The only other Christmas reference I remember that made it on air is that at the end of one of the episodes (I just saw this on one of my tapes), I think just before the arrival of Laura (the Phoenix) in Collinsport, during the closing credits the announcer says something to the effect that "The producers and staff of Dark Shadows wish you and your family a very happy holiday season."
What episode number was this?
Sorry this is a bit late and not quite so interesting now that the holidays are over (unless you're an Orthodox Christian perhaps).
I believe it was episode #126. [spoiler]Burke and Roger rescue Vicki at the Old House where she has been held captive by Matthew Morgan. Matthew was about to kill Vicki when ghosts called him from the hiding place behind the bookcase to the parlour.[/spoiler]
At the very end of the credits, the announcer says:
"On behalf of our sponsors, we would like to wish you a very happy holiday season. Dark Shadows is a Dan Curtis production."