No.17874
Series with Christmas as a focus or major plot point:
Amagami SS
Itsudatte My Santa! (2 episode OVA)
White Album (this anime is shit)
Toradora
Kurayami Santa (not about Christmas but the main character is named Santa)
Films
The Disappearance of Haruhi Suzumiya (December 16-21, /a/'s annual viewings are on the 18th)
Tokyo Godfathers
Santa Company
Individual episodes/Christmas specials:
Love Hina Christmas Special
School Rumble S2 episode 19
Lucky Star episode 11
Azumanga Daioh episode 17
Minami-ke episode 12
K-On episode 7
Nadesico episode 13
Vandread episode 10
The Big O episode 11
Acchi Kocchi episode 10
Gochuumon wa Usagi desu ka? episode 11
Guilty Crown: Lost Christmas (not really)
Baka to Test Christmas special
Polar Bear Cafe episode 37
Zan Sayonara Zetsubou Sensei episode 13
Rokujouma no Shinryakusha episode 12
Locodol OVA 2
Fushigi na Somera-chan episode 10
Osomatsu-san episode 11
Bakuon!! episode 8
Sansha Sanyou episode 10
momokuri episodes 23-24
No.17879
Came up with a few more things since I posted the list to 4chan /a/. Gintama 37+200 are Christmas episodes. Gundam 0080 takes place during Christmas, and ef - A Tale of Memories starts at Christmas. Also, while not Christmas-themed, Sora no Woto, Haibane Renmei, Spice & Wolf, and Mushishi all have winter motifs and are good for watching during the snowy season.
No.17880
The other magical girl in Mahou Shoujo Nante Deskar wears a Christmas outfit.
No.17882
>>17881A quick search gives me episodes 12, 62, 114, and 165.
More additions to the list: the Di Gi Charat Christmas Special, and Hidmari Sketch episode 12.