Definitely Mrs. Santa Claus. It came out when I was about ten years old. IMDB Precis:
Neglected by her husband during the pre-Christmas rush, Mrs. Santa Claus takes the reindeer and sleigh out for a drive, only to end up stranded in the multi-cultural neighbourhood of Manhattan's Lower East Side of the early 1900s.
It stars Angela Lansbury as Mrs. C and has a delightfully camp Terrence Mann as the baddy toymaker, who wants everyone to have an awful Christmas because his brother stole his toy and broke it when he was a child. The music and choreography is wonderful and to my musical-loving ten-year-old self, it was amazing. And my musical-loving 24-year-old self would happily watch it again. Again.