Well first and most importantly, I'm not sure how to take your argument as I never made any statement one way or another on what seemingly seems to be a rebuttal using a negative as a form of correction.

It's a western Christian holiday and the US is largely a country settled by western Christians in the early days, so most of the US follows the holiday. That makes it an American holiday in my book, although the US is by far not the only ones that follow it or the origin of the holiday.

The US has put its own spin on how the holiday is celebrated and spread those customs to other parts of the world too, so the topic is a bit less white or black in that there is a two-way spread of cultural customs.