The new United States celebrates its first national day of thanksgiving on Thursday, December 18, 1777, commemorating the American victory at the Battle of Saratoga after the surrender of General John Burgoyne and 5,000 British troops in October 1777.
In proclaiming the first national day of thanksgiving, Congress wrote, "It is therefore recommended to the Legislative or executive Powers of these UNITED STATES, to set apart THURSDAY, the eighteenth Day of December next, for solemn THANKSGIVING and PRAISE; That at one Time and with one Voice the good People may express the grateful Feelings of their Hearts, and consecrate themselves to the Service of their Divine Benefactor"
Neither when the Congress proclaimed the day of Thanksgiving on November 1, nor when the population celebrated in December, were they aware that on December 17, the French would finally formalize a military and trade alliance with the rebelling states. These were not disconnected events. The victory at Saratoga convinced the French king that the Americans might be worthy allies and the ensuing alliance made an American victory possible.
Merely having a national day of thanksgiving was a tremendous step forward in creating an American identity. Previously, the colonies had celebrated individually or as part of the British Empire. Now they had experienced an event that had affected them all and formalized a celebration that involved them all. With the French alliance, they had an ally who supported them all. Americans had just taken a major step on the tortured trail from colonies to states and from states to nation.