Considering we're in the Festive Season at the moment, it got me thinking about Christmas. I don't recall any 'Christmas specials' in any Star Trek series, so my question is: is Christmas referenced in any of the Star Trek series in such a way that implies they actually do still observe Christmas in both (or either) the 23rd and/or 24th centuries? This therefore excludes Enterprise as that is set in the 22nd century, not the 23rd or 24th century.
23rd Century - Yup
BERKELEY: Locked in, sir. Ready to beam down.
NOEL: Doctor Helen Noel, Captain. We've met. Don't you remember the science lab Christmas party?
KIRK: Yes, I remember.
TOS: Dagger of the Mind
24th Century - Yup
CHAKOTAY: I already gave them a Voyager medallion. They seemed appreciative.
JANEWAY: Come in. Doesn't it look like Christmas morning in here, Commander?
CHAKOTAY: You have to admit the generosity of our guests is very impressive.
VOY: Survival Instinct
And moving down to the (frankly laughable) EU, we see the Enterprise crew enjoying an xmas skiing celebration on the holodeck in TNG: Spirit in the Sky!
Not exactly the best answer, but an indication that I've just remembered whilst writing this up was a scene from Star Trek Generations whilst Picard was in the Nexus:
The Christmas tree and other decorations show conclusively that Picard is in a place where Christmas is being celebrated. Judging by the clothing though, it doesn't look like the 23rd or 24th centuries to me though.
This provides some indication that Christmas may have been celebrated in the 24th century. However, Picard actually isn't in the 24th (or 23rd) century at that time.
If another answer provides more definitive proof of Christmas being celebrated in the 23rd or 24th century, that will be a more suitable answer for this question.