The idea of creating a formal national Thanksgiving Day holiday originated with Illinois born President Abraham Lincoln?
 
In an 1863 proclamation – amid the still-raging Civil War – Lincoln designated Thanksgiving to be celebrated on the fourth Thursday of November.
 
"He was always looking for ways to unify the nation in a terrible time of war," said Lincoln biographer Ronald C. White Jr.

Learn more:
Official Thanksgiving Holiday - USA Today
Thanksgiving in the United States