You'll Have to Narrow it Down
The great thing about this is that there are so many variations on this theme! You'll have to narrow it down to a smaller niche. Golden Age Christmas covers is probably still too big a category. Perhaps, Disney Christmas train covers, for example. Even just a cursory search for Christmas covers turned up issues from every conceivable universe and genre. Check out this awesome Witching Hour #28 for example. So horror/Christmas is an option. Good to know. I really think the more specialized your niche, the more fun you'll have with these.
Christmas Special or Regular Issue?
Another option is to collect holiday special issues, these are generally one shots, though they might have a run of their own. For example, Archie's Christmas Stocking. There are at least seven issues of this spread throughout the 1990s. In addition to these special issues, often publishers released regular monthly titles with a Christmas theme (like the aforementioned Witching Hour). Batman #27 from 1945 is another good example.
A collector might find just tracking down the holiday covers of a favorite character will be quite the project. Even newer creations like Gwenpool have multiple holiday themed covers out there. How about this Gwenpool Holiday Special variant, is that Miles? Don't tell me Galactus met Santa Claus!
It's All Good
Sometimes it is easy to lose sight of why we love this hobby so much as we bid up those keys and search for the next big hitter. What better time to take a deep breath and discover some amazing covers featuring holiday themes than right now as we prepare for the season of giving? Put together a fun combo like Betty & Veronica Christmas tree covers, or DC Santa Claus covers. Maybe Marvel Outlawed characters in a holiday setting? The possibilities are virtually limitless.
Find Christmas Comic Covers in the Wild?
Normally, this is a great category to narrow down and try to find in the wild. With conventions still on hold and comic shops limiting access, that might be difficult for awhile. But check in with your local shop, ask if they have some holiday books buried in the boxes. The shop and its customers might appreciate the nudge. Many shops are doing online, live sales (Facebook and so on), might be nice to get some holiday covers in the mix!
Make a Fantastic Holiday Display
These covers can make a great seasonal display too. As we all know, a big part of the attraction of comics is nostalgia. Some well placed comic books on the wall or on a table display might reach someone who normally isn't a "comic person." 
