[Picture: Surrender of General Burgoyne at Saratoga]
In the 1700's individual colonies would periodically designate a day of thanksgiving in honor of a military victory, an adoption of a state constitution or an exceptionally bountiful crop.
During the American Revolutionary War the Continental Congress appointed one or more thanksgiving days each year, each time recommending to the executives of the various states the observance of these days in their states. The First National Proclamation of Thanksgiving was given by the Continental Congress in 1777.
George Washington, leader of the revolutionary forces in the American Revolutionary War, proclaimed a Thanksgiving in December 1777 as a victory celebration honoring the defeat of the British and the surrender of British General Burgoyne at Saratoga.