By "Christmas star" do you mean the Jupiter & Saturn thing?
On December 21 they will align, from the POV of earth observers, to the extent that they will basically appear like one big shiny thing instead of 2 big shiny things.
They align somewhat like that once every twenty years, but align-close-enough-to;look-like-one-thing only occurs once every few centuries.
Did that happen around "Year 0"? I didn't see any research indicating that is the case, but I didn't spend much time on it so maybe I missed it.