I'm peripherally aware it has something to do with (North American first nation) Indians and early settlers plus it tends to involve feasting on turkey and pumpkin dishes.
As far as the US holiday is concerned, it's secular, by which I mean it is celebrated by pretty much everybody regardless of religious persuasion (or lack thereof).
That said, not all American Jews do view Thanksgiving as acceptable. Among ultra-Orthodox Jews, the belief is widespread that celebrating Thanksgiving violates Jewish religious law against observing non-Jewish festivals. Likewise, some conservative Muslims (Salafis in particular) view Thanksgiving as haram for parallel reasons. You might say these Jewish and Muslim objectors are paying more attention to the festival’s Christian origins than most Americans do.
Thanksgiving isn’t the only example of a minor American Christian tradition being secularised - the same is true of Mother’s Day and Father’s Day. Unlike Thanksgiving, which is rarely observed outside of North America, Mother’s Day and Father’s Day have spread to most of the rest of the English-speaking world, even if not always on the same date. Many cultures have indigenous traditions of festivals to celebrate motherhood and/or fatherhood, going back centuries or more; but in Anglophone countries, Mother’s Day and Father’s Day are generally American in origin, and only date to the 20th century, not due to one of those older traditions.
It is Christian only in the sense that the people who celebrated it were Christian. The events that led to the festival were purely political in nature, and not necessarily wholesome either.
1. https://www.theatlantic.com/ideas/archive/2019/11/thanksgivi...
2. https://www.salon.com/2016/11/23/thanksgiving-a-day-of-mourn...
US currency in itself isn't inherently religious, but the decision to put that phrase on the currency was religious, and so is the decision to keep it there today. Now, it is not just religious, it is also very political, but the two are very often intertwined: it is a form of politicised religion, or religious politics.
> So I'm not sure the proclamation makes Thanksgiving religious. The origins might, though
I think to most Americans in the 19th century and earlier, the idea that Thanksgiving was a religious festival would have seemed obvious–it was about giving thanks to the deity, assuming a Judaeo-Christian conception of deity. Now, no denying that it became quite secularised through the course of the 20th century, and to many 21st Americans it is an entirely secular occasion, and if "thanksgiving" is anything more than an empty word, it is thanks directed at one's friends/family/colleagues/acquaintances/community/etc, maybe even at the cosmos, but not at God in whom one quite possibly doesn't believe.
As I said in my original comment, other religious festivals, such as Christmas, have also become highly secularised. But, even though many celebrate Christmas in an entirely secular way, people still remember its association with Christianity, which makes many non-Christians feel uncomfortable celebrating it even in a secular form. Thanksgiving was never so explicitly Christian, so Jews and Muslims and others feel more comfortable in celebrating it.
We dont have it here in Australia (except apparently on a small island off to the side of the mainland). We don't really have any sort of festival or celebration that is comparable.
Of course it is not entirely equivalent, in that Thanksgiving is a more religiously inclusive holiday than Christmas is. While there are people with a religious objection to both, there are many more with a religious objection to the later but not the former.
I think this recent adoption of Black Friday in Australia is dumb, for two reasons:
(1) It makes zero sense given we don’t celebrate Thanksgiving
(2) Given the term’s historical association in Australian culture with mass death (1939 Black Friday bushfires that killed over 70 people, 2009 Black Saturday bushfires that killed over 170 people), using it for sales could be seen as disrespectful and culturally insensitive
Much further away from the equator, you can't really do agriculture to harvest stuff. Closer to the equator you can often do multiple harvests a year.
Mentioning this not just to be pedantic (though I enjoy that), but also because the connection between our physical surroundings and culture are endlessly fascinating.