It's "La Befana" by Gene Wolfe.
“La Befana.” An old woman comes to a hostile planet in search of redemption for her sins, committed long, long ago.
Some quotes from the story:
When Zozz, home from the pit, had licked his fur clean, he howled before
John Bannano's door. John's wife, Teresa, opened it and let him in. She
was a thin, stooped woman of thirty or thirty-five, her black hair shot with
gray; she did not smile, but he felt somehow that she was glad to see him.
She said, "He's not home yet. If you want to come in we've got a fire."
Bananas nodded. Maria said, "Daddy, how did Grandmother come
"Same way we did."
Mark said, "You mean she signed a thing?"
"A labor contract? No, she's too old. She bought a ticket - you know, like
you would buy something in a store."
Maria said, "That's what I mean."
"Shut up and play. Don't bother us."
Zozz said, "How'd things go at work?"
"So-so." Bananas looked toward the back room again. "She came into
some money, but that's her busines - I didn't want to talk to the kids about
"She says she spent every dollar to get here - you know, they haven't
used dollars even on Earth for fifty, sixty years, but she still says it, how do
you like that?" He laughed, and Zozz laughed too. "I asked how she was
going to get back, and she said she's not going back, she's going to die
right here with us. What could I say?"
"She could have lived good for the rest of her life on what that ticket cost
her." Bananas was silent for a moment. "She used to be a big, fat woman
when I was a kid, you know? A great big woman with a loud voice. Look at
her now - dried up and bent over; it's like she wasn't my mother at all. You
know the only thing that's the same about her? That black dress. That's the
only thing I recognize, the only thing that hasn't changed. She could be a
stranger - she tells stories about me I don't remember at all."
Maria said, "She told us a story today."
Mark added: "Before you came home. About this witch."
Maria said: "That brings the presents to children. Her name is La Befana
the Christmas Witch."
Zozz drew his lips back from his double canines and jiggled his head. "I
The twist, which the querent did not include, is:
Grandmother is implied to be "La Befana", doomed to walk the world seeking the Christ child, here even to other worlds, although the last line implies her journey is almost at an end. As Russel hinted in his comment, the last line is, "Not forever, dearest; only until tomorrow night."