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):
Honoring the Mothers
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.
Here is a modern invocation for Holda by Diana Paxson:
Holy Holla, in the heavens,
A snowy featherbed you’re shaking –
Bless the earth with your white blanket,
Moist the mantle you are making.
Holla high above come riding.
Your Wagon rolls through winter weather;
Turn away your face of terror,
Bless us as we bide together.
Holy Holla, here we gather,
Send us skill in all our spinning.
Huldrefold (Gnomes) to help in housework,
Wealth and health with your aid winning.