I have read it many times and I agree that the story is about a dream/nightmare Ebeneezer has on Christmas Eve. I always saw the story as an old rich man had who does not have much longer to live reflects on his life. He decides he still has time to try and change with what little time he has left.

I think it really could have been framed many ways. Many people reflect on their lives on New Year’s Eve or their birthdays. I think Dickens just decided to choose Christmas for his story.

EDIT: In short, man nearing end of life has existential crisis in dream.