"Catspaw" is apparently the only so-called "holiday special" episode of Trek, and that for Halloween. We've had some mentions of Christmas, but no real Christmases have been shown (Picard's fantasy Christmas in the Nexus doesn't count.
If TOS/TAS had done a Christmas-themed episode, I imagine it might have been along the lines of discovering Santa was a benevolent alien/Human who lived a long time ago (a la Flint) and his elves were aliens. He would need help if someone kidnapped the elves as slave labor or some such thing.
For Valentine's, I suppose, any love-themed episode will do. New Years doesn't mean much when you're not on Earth, or if you deal with races who all have different new years days.
If TOS/TAS had done a Christmas-themed episode, I imagine it might have been along the lines of discovering Santa was a benevolent alien/Human who lived a long time ago (a la Flint) and his elves were aliens. He would need help if someone kidnapped the elves as slave labor or some such thing.
For Valentine's, I suppose, any love-themed episode will do. New Years doesn't mean much when you're not on Earth, or if you deal with races who all have different new years days.