Mother Night is traditionally celebrated on the third day of the twelve days of Christmas. I couldn't find much lore, but I did find this nice little article in The Winter Solstice about Mother Christmas (Frau Holle, Holda, Hulda, Holla):
There are so many male figures associated with the Winter Solstice, we should give room to at least one female character, and who better than Mother Christmas? On this day, include a figure of Holda upon your Winter shrine....
I am so sorry to do this to you, but this post has been moved to my new website, The Pagan Calendar (hosted at shirleytwofeathers.com) and can be found in its entirety here: Mother Night
As you explore this site, you may find links to a "page not found" instead of something cool and magickal. For this I apologize. I am very working hard behind the scenes to restore those pages along with a link to their homes on my new website where they can be viewed in full.