Everyone thinks that the US holiday of Thanksgiving is due to a dinner of Pilgrims with Native Americans. Though there are some vague links in terms of occasional days declared as days for giving thanks, the national holiday of Thanksgiving does not date until the US Civil War, in 1863. As part of an effort to create a Union-wide anti-Slavery holiday, Abraham Lincoln first declared the last Thursday in November as a national and annual holiday. Rather than a day in which an idyllic meeting of cultures before they turned blood-thirsty was celebrated, it was a day established in the fight to hold a country together which could not agree on the legality of slavery.
This makes Thanksgiving a very good example of how national history gets mythologized in political memory and secular ritual.