Well I think there is no date at which it is appropriate to post a thread about Xmas etiquette, as it is, in my view, a nonsensical subject.
However, I do understand that, culturally speaking, some may find such a thread useful or even necessary, if they want to 'fit in' with those around them.
So, to answer your Q, if push comes to shove, and balancing the weight of Father Christmas against that of Jultomte, I'd say 23rd December, or a day ot two earlier if that date falls on a Saturday or Sunday, or if you work awkward shifts.