Simply put: Thanksgiving is the day when the dominant white culture (and, sadly, most of the rest of the non-white but non-indigenous population) celebrates the beginning of a genocide that was, in fact, blessed by the men we hold up as our heroic founding fathers.
The first president, George Washington, in 1783 said he preferred buying Indians' land rather than driving them off it because that was like driving "wild beasts" from the forest. He compared Indians to wolves, "both being beasts of prey, tho' they differ in shape."
Thomas Jefferson -- president #3 and author of the Declaration of Independence, which refers to Indians as the "merciless Indian Savages" -- was known to romanticize Indians and their culture, but that didn't stop him in 1807 from writing to his secretary of war that in a coming conflict with certain tribes, "[W]e shall destroy all of them."
As the genocide was winding down in the early 20th century, Theodore Roosevelt (president #26) defended the expansion of whites across the continent as an inevitable process "due solely to the power of the mighty civilized races which have not lost the fighting instinct, and which by their expansion are gradually bringing peace into the red wastes where the barbarian peoples of the world hold sway."
Roosevelt also once said, "I don't go so far as to think that the only good Indians are dead Indians, but I believe nine out of ten are, and I shouldn't like to inquire too closely into the case of the tenth."
How does a country deal with the fact that some of its most revered historical figures had certain moral values and political views virtually identical to Nazis? Here's how "respectable" politicians, pundits, and professors play the game: When invoking a grand and glorious aspect of our past, then history is all-important. We are told how crucial it is for people to know history, and there is much hand wringing about the younger generations' lack of knowledge about, and respect for, that history.