We know that the Potterians (Potterites?, Potterlings?) celebrate Christmas (at least they do in the UK).
Are there mentions of Santa? Is/was he some sort of real person. In the same way we (out of universe) consider Merlin to be fictional, JK has co-opted him as a historical figure (in universe ).
He is certainly mentioned but this doesn't address whether he's 'real'.
Somewhat related, but doesn't answer my question: Did J. K. Rowling's Religious Beliefs Determine Much in Harry Potter?