From our most respected founding father, George Washington – Transcript of President George Washington’s Thanksgiving Proclamation from October 3, 1789:
By the President of the United States of America—
A Proclamation

Now therefore I do recommend and assign Thursday the 26th day of November next to be devoted by the People of these States to the service of that great and glorious Being, who is the beneficent Author of all the good that was, that is, or that will be—That we may then all unite in rendering unto him our sincere and humble thanks—for his kind care and protection of the People of this country previous to their becoming a Nation—for the signal and manifold mercies, and the favorable interpositions of his providence, which we experienced in the course and conclusion of the late war—for the great degree of tranquility, union, and plenty, which we have since enjoyed—for the peaceable and rational manner in which we have been enabled to establish constitutions of government for our safety and happiness, and particularly the national One now lately instituted, for the civil and religious liberty with which we are blessed, and the means we have of acquiring and diffusing useful knowledge; and in general for all the great and various favors which he hath been pleased to confer upon us.

Given under my hand at the City of New York the third day of October in the year of our Lord 1789.
Go. Washington