Versions of what we call Thanksgiving have been celebrated for thousands of years. Harvest festivals, feasts--these must be as old as civilization itself. Even our particular habit of offering thanks is hardly new; in the high-mortality, season-by-season existence of the old days, offering thanks was nothing special. Cool, no Black Death this year!
This makes it the most univeral of holidays, moored to no specific culture or religious tradition--and maybe the most American. At least in our current version of the Thanksgiving story, we tell the fable of Europeans and Native Americans breaking bread in cooperation. It's an immigrant story, and a story of proto-democracy. It is a template for the national myth.
Since, by habit, we cite our specific thanks, let me make that mine. No one gets to claim Thanksgiving or mentally exclude others. It's one of the very few times of year when the identity of "American" muscles others out of the way. I'm thankful for the moment to enjoy my whole country, north to south, red to blue.
That and the beer. But I'm always thankful for that.