Lies My Teacher Told Me by James Loewen, 1995, Excerpts
The true history of Thanksgiving reveals embarrassing facts. The Pilgrims did not introduce the tradition; Eastern Indians had observed autumnal harvest celebrations for centuries. Our modern celebrations date back only to 1863. During the Civil War, when the Union needed all the patriotism that such an observance might muster, Abraham Lincoln proclaimed Thanksgiving a national holiday. The Pilgrims had nothing to do with it; not until the 1890s did they even get included in the tradition.
The First Thanksgiving  by Jennie A. Brownscombe (1850-1936), Pilgrim Hall Museum, Plymouth, Massachusetts