Prove "Santa is a metaphorical descriptor for the Christmas spirt"
You have to understand 'spirit' as a form of culture, i.e. circulating patterns of subjective form.
Forget about the metaphysical and supernatural for a moment and just understand 'spirits' as patterns of behavior, thought, feelings, etc.
Then you should be able to understand what "Christmas spirit' is, how it comes over people and motivates them to visit each other, exchange gifts, sing carols, watch Christmas movies, etc. etc.
Literally, 'spirit' is anything that shapes subjectivity. 'Christmas spirit' and 'school spirit' are just two very common examples. You could say there is a spirit of war, for example, or a spirit of sincerity, sarcasm, hard work, hedonism, laziness, brotherly love, charity, discipline, etc. etc.
Just look at all the patterns (social) subjectivity can take and consider the complex networks of events that happen to cause those patterns to take form in various ways, and reproduce themselves through memories, habits, traditions, rituals, etc.