Historians agree that the Pilgrims really did celebrate a first Thanksgiving. But it wasn’t turned into a regular yearly celebration until Abraham Lincoln made it official during the middle of the Civil War, some 250 years later. New documents have come to light that may explain why. “Never again,” writes John Alden in a letter found in a newly discovered cache of papers composed by the original passengers of the Mayflower. “Six long hours we have spent looking at the hind end of a horse on the overly crowded road to the house of my parents and lo, for what? …