On 25 September 1789, Elias Boudinot of Burlington, New Jersey, introduced in the United States House of Representatives a resolution “That a joint committee of both Houses be directed to wait upon the President of the United States, to request that he would recommend to the people of the United States a day of public thanksgiving and prayer to be observed by acknowledging, with grateful hearts, the many signal favors of Almighty God, especially by affording them an opportunity peaceably to establish a Constitution of government for their safety and happiness.” The House was not unanimous in its determination to give thanks. Aedanus Burke of South Carolina objected that he “did not like this mimicking of European customs, where they made a mere mockery of thanksgivings.” Thomas Tudor Tucker “thought the House had no business to interfere in a matter which did not concern them. Why should the President direct the people to do what, perhaps, they have no mind to do? They may not be inclined to return thanks for a Constitution until they have experienced that it promotes their safety and happiness. We do not yet know but they may have reason to be dissatisfied with the effects it has already produced; but whether this be so or not, it is a business with which Congress have nothing to do; it is a religious matter, and, as such, is proscribed to us. If a day of thanksgiving must take place, let it be done by the authority of the several States.” 
Citing biblical precedents and resolutions of the Continental Congress, the proponents of a Thanksgiving celebration prevailed, and the House appointed a committee consisting of Elias Boudinot, Roger Sherman, and Peter Silvester to approach President Washington. The Senate agreed to the resolution on 26 September and appointed William Samuel Johnson and Ralph Izard to the joint committee. On 28 September the Senate committee reported that they had laid the resolution before the president.  Washington issued the proclamation on 3 October, designating a day of prayer and thanksgiving.
Whatever reservations may have been held by some public officials, the day was widely celebrated throughout the nation. The Virginia assembly, for example, resolved on 19 November that the chaplain “to this House, be accordingly requested to perform divine service, and to preach a sermon in the Capitol, before the General Assembly, suitable to the importance and solemnity of the occasion, on the said 26th day of November.”  Most newspapers printed the proclamation and announced plans for public functions in honor of the day. Many churches celebrated the occasions by soliciting donations for the poor. Washington’s secretary, Tobias Lear, wrote to John Rodgers, pastor of the two Presbyterian churches in New York City, on 28 November, that “by direction of the President of the United States I have the pleasure to send you twenty five dollars to be applied towards relieving the poor of the Presbyterian Churches. A paragraph in the papers mentioned that a contribution would be made for that purpose on Thanksgiving day; as no opportunity offered of doing it at that time, and not knowing into whose hands the money should be lodged which might be given afterwards–The President of the United States has directed me to send it to you, requesting that you will be so good as to put it into the way of answering the charitable purpose for which it is intended.” 
Washington enclosed the Thanksgiving Proclamation in his Circular to the Governors of the States, written at New York on 3 October 1789: “I do myself the honor to enclose to your Excellency a Proclamation for a general Thanksgiving which I must request the favor of you to have published and made known in your State in the way and manner that shall be most agreeable to yourself.” 
The above is adapted from the annotation to Washington’s Circular to the Governors of the States, 3 October 1789, printed in volume 4 of The Papers of George Washington, Presidential Series, W. W. Abbot, Dorothy Twohig, et al (University Press of Virginia, Charlottesville and London, 1993; Dorothy Twohig, volume editor), pp. 129-30.
1. Joseph Gales, Sr., compiler. The Debates and Proceedings in the Congress of the United States; with an Appendix, Containing Important State Papers and Public Documents, and All the Laws of a Public Nature. (Annals of Congress.) 42 vols. Washington, D.C., 1834-1856, pp. 1:949-50.
3. Journal of the House of Delegates of the Commonwealth of Virginia; Begun and Holden in the City of Richmond . . . on Monday, the Nineteenth Day of October, in the Year of Our Lord One Thousand Seven Hundred and Eighty-nine. Richmond, 1828, p. 70.
Presidential Series, Volume 4
The original document used here online is the Library of Congress copy (DS, DLC:GW) of the Thanksgiving Proclamation.