Nov. 26, 1789. President Washington Proclaimed Thanksgiving Day. This was passed by both the Houses of Congress. However, the first Thanksgiving proclamation was given not in fall but summer... on June 20, 1676.

The proclamation is as follows:

Nov 26, 1789
Whereas it is the duty of all Nations to acknowledge the providence of Almighty God, to obey His will, to be grateful for His benefits, and humbly to implore His protection and favor, and Whereas both Houses of Congress have by their joint committee requested me to recommend to the People of the United States a day of public thanks-giving and prayer to be observed by acknowledging with grateful hearts the many single favors of Almighty God, especially by affording them an opportunity peaceably to establish a form of government for their safety and happiness.

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 single 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, of the civil and religious liberty with which we are blessed, and the means we have to acquiring and diffusing useful knowledge and in general for all the great and various favors which He hath been pleased to confer upon us.