- Kodomo no Omocha #38: "Dokidoki Kiss for Christmas"
- Ranma 1/2 OVA 2, "Tendo Family Christmas Scramble"
- Urusei Yatsura #10 "Pitter Patter Christmas Eve"
- ToHeart #12 "Season of Feelings" and #13 "A Snowy Day"
- Kimagure Orange Road #38 "Kyousuke Timetrips! The Third Christmas!"
- Marmalade Boy #40 "A Holy Night / I'm Alone on Christmas Eve!" and #41, "The Morning of Lovers / Merry Christmas!"
Popotan has a Christmas-themed episode, although it's not actually Christmas Day when the episode takes place. The magical moving house also contains a shop which sells Christmas-themed merchandise.
My "Big Christmas Anime Tape" contains other episodes besides the ones listed above: The Love Hina Christmas Special and Hime-chan's Ribbon #13 "SMAP Came". The latter merely takes place on Christmas Day at a school Christmas festival. The Love Hina special is...well...it's Love Hina which is--IMHO--the nadir of the harem comedy genre.
Maison Ikkoku's first season contains two Christmas episodes. Tenchi Muyo! Daughter of Darkness takes place around Christmas and revolves around the Juraian holiday of Startica. Oh! My Goddess! OVA episodes 4 and 5 are set during Christmastime, if you can handle 400-proof angst. (Yes, I know "400 proof" is 200% alcohol.)
There are other series which have Christmas-themed episodes. There are plenty of them; the Japanese celebrate Christmas as a secular holiday. (A minority of Japanese are Christian and celebrate Christmas as a holy day.)
So, if you've gotten tired of Charlie Brown Christmas, Rudolph the Red-nosed Reindeer, and The Year Without a Santa Claus, there are alternatives.