This book must date back to the 1940s. I remember finding it at our summer home in NH, where there were all kinds of odd books for young people about.
It is a picture book. The illustrations are similar to pen-and-ink drawings that have been colored in, quite bright, beautiful sense of line.
The story is about a girl (and her brother) in a northern country (Scandinavia?). In the winter (Christmas?) they decorate sleds with strings of colored tassels for some procession. The girl cleans house for an old woman who makes her the tassel string. The girl gets caught in a terrible snowstorm on the way home and takes shelter in the root-hole of a much loved ancient oak tree. (The oak tree is a character in the story too.) This is where she is found.
I have never encountered this book anywhere else. It doesn’t ring a bell with anyone I have ever talked to.
Thanks for your suggestions!