So many regular series, since the dawn of time, have had Christmas episodes, which I don't consider "specials." Still, I suppose I might as well at least list some of them, in passing. At least the ones I personally remember (even vaguely) seeing; surely there have been many I've seen that I've forgotten, and many more that I've never seen at all. Also note that some shows, particularly animated ones, have episodes broken into separate shorts, not all of which will necessarily be related to the holiday. (This section is for not only Christmas, but potentially Hanukkah or any other wintery/December holidays, including fake ones.) Oh, and while the holiday may play a particularly prominent role in many of the episodes listed here, sometimes it's only a very minor aspect of the episode's plot.
= My favorite Christmas episodes. (Some eps that I don't remember well might become favorites if I ever see them again.)
Sabrina, the Teenage Witch
Season 1: A Girl and Her Cat (Amazon; Christmas Specials Wiki; IMDb; Paramount+; Wikia)
Season 2: Sabrina Claus (Amazon; Christmas Specials Wiki; IMDb; Paramount+; Wikia)
Saturday Night Live
There have been many episodes, over the years, that contained Christmas or Hanukkah sketches. The ones I'd know would mostly be from the 90s, but I'm sure they started doing such things in the 70s, and still do them now. And occasionally they might air a compilation episode of such sketches from over the years. (I do have one sketch listed on my It's a Wonderful Life parodies page and another on my Gift of the Magi parodies page.)
The Secret World of Alex Mack
Season 2: The Gift (Amazon; Christmas Specials Wiki; IMDb)
Seinfeld
Season 9: The Strike (A.V. Club; Christmas Specials Wiki; IMDb; Wikia; Wikipedia) Other episodes have at least mentioned Christmas or Hanukkah, but this ep is famous for introducing the holiday "Festivus."
The Simpsons
Season 1: Simpsons Roasting on an Open Fire (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 7: Marge Be Not Proud (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 9: Miracle on Evergreen Terrace (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 11: Grift of the Magi (Amazon; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 15: 'Tis the Fifteenth Season (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
(The above has both a Christmas Carol parody and a Grinch parody.)
Season 17: Simpsons Christmas Stories (Amazon; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 22: The Fight Before Christmas (Amazon; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 25: White Christmas Blues (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Season 26: I Won't Be Home for Christmas (Amazon; A.V. Club; Christmas Specials Wiki; Disney+; IMDb; TV Tropes; Wikia; Wikipedia)
Single Parents
Season 1: The Magic Box (Amazon; Christmas Specials Wiki; IMDb; Wikia)
Season 2: Good Holidays to You (Amazon; Christmas Specials Wiki; IMDb)
Smallville
Season 5: Lexmas (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia)
So Weird
Season 2: Fountain (IMDb; This Was Television)
Son of Zorn
Season 1: The War on Grafelnik (Amazon; A.V. Club; IMDb)
South Park
1992/95 shorts: The Spirit of Christmas
1997 short: Cartman Sings O Holy Night (no links available) He would later sing it again in the season 3 ep "Mr. Hankey's Christmas Classics."
Season 1: Mr. Hankey, the Christmas Poo (Amazon; A.V. Club; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 2: Merry Christmas, Charlie Manson! (Amazon; A.V. Club; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 3: Mr. Hankey's Christmas Classics (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 4: A Very Crappy Christmas (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 6: Red Sleigh Down (Amazon; Christamas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 7: It's Christmas in Canada (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Season 8: Woodland Critter Christmas (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
The Spectacular Spider-Man
Season 2: Reinforcement (Christmas Specials Wiki; IMDb; Marvel Animated Wiki; TV Tropes)
Speechless
Season 1: C-h-o-- Choir (Amazon; IMDb; Wikia)
Season 2: S-i-- Silent N-i-- Night (Amazon; IMDb; Wikia)
Season 3: J-i-- Jingle T-h-- Thon (Amazon; Christmas Specials Wiki; IMDb; Wikia)
Sports Night
Season 2: The Reunion (Amazon; A.V. Club; IMDb; TV Tropes; Wikia)
Static Shock
Season 2: Frozen Out (Christmas Specials Wiki; DCAU Wiki; IMDb; Wikia; World's Finest)
Studio 60 on the Sunset Strip
Season 1: The Christmas Show (Amazon; IMDb)
Suburgatory
Season 1: The Nutcracker (Amazon; A.V. Club; IMDb; Wikia)
Season 2: Krampus (Amazon; A.V. Club; IMDb; Wikia)
Supernatural
Season 3: A Very Supernatural Christmas (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
The Tick
Season 2: The Tick Loves Santa! (Christmas Specials Wiki; IMDb)
Transformers Animated
Season 3: Human Error (IMDb; TransformersAnimated.com; Wikia)
The Troop
Season 1: The Good, the Bad, and the Ickie Doll (Amazon; Christmas Specials Wiki; IMDb; Wikia)
The Twilight Zone
Season 2: The Night of the Meek (Amazon; A.V. Club; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Veronica Mars
Season 1: An Echolls Family Christmas (Amazon; A.V. Club; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
Voyagers!
Season 1: Merry Christmas, Bogg (Amazon; IMDb; TV Tropes)
Warehouse 13
Between seasons 2 and 3: Secret Santa (Amazon; IMDb; Syfy; Wikia)
Between seasons 3 and 4: The Greatest Gift (Amazon; IMDb; Syfy; Wikia)
The Weekenders
Season 3: The Worst Holiday Ever (Christmas Specials Wiki; IMDb; Wikia)
Wonder Pets!
Season 1: Save the Reindeer! (Amazon; Christmas Specials Wiki; IMDb; Wikia)
Season 2: Save the Nutcracker! (Amazon; Christas Specials Wiki; IMDb; Wikia)
The Wonder Years (2021)
Season 1: Home for Christmas (Amazon; Christmas Specials Wiki; IMDb; Wikia)
The X-Files
Season 5: Christmas Carol (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia) (first episode of a two-parter)
Season 6: How the Ghosts Stole Christmas (Amazon; Christmas Specials Wiki; IMDb; TV Tropes; Wikia; Wikipedia)
X-Men: Evolution
Season 2: On Angel's Wings (Amazon; Christmas Specials Wiki; IMDb; Wikia)