Schoolchildren all over the United States know the story: the Pilgrims landed at Plymouth Rock and shared in the bountiful harvest of the Wompanoag Native Americans back in 1621.  

Since then, Americans have been giving thanks at this time of year.  And since President Roosevelt moved the holiday to the fourth Thursday in November in 1941 (the holiday itself being proclaimed by President Lincoln nearly 80 years prior), we have celebrated Thanksgiving on this day.