An original and thus hard-to-read version of George Washington's Thanksgiving proclamation can be viewed through the Library of Congress website. The economist in me was intrigued to notice that some of the causes for giving of thanks included "the means we have of acquiring and diffusing useful knowledge ... the encrease of science among them and us—and generally to grant unto all Mankind such a degree of temporal prosperity as he alone knows to be best."
Also, the original Thankgiving proclamation was not without some controversy and dissent in the House of Representatives, as reported by the Papers of George Washington website at the University of Virginia.
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.”
Here's the transcript of George Washington's Thanksgiving proclamation from the National Archives.
By the President of the United States of America. a Proclamation.
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 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 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 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 tranquillity, 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.
and also that we may then unite in most humbly offering our prayers and supplications to the great Lord and Ruler of Nations and beseech him to pardon our national and other transgressions—to enable us all, whether in public or private stations, to perform our several and relative duties properly and punctually—to render our national government a blessing to all the people, by constantly being a Government of wise, just, and constitutional laws, discreetly and faithfully executed and obeyed—to protect and guide all Sovereigns and Nations (especially such as have shewn kindness unto us) and to bless them with good government, peace, and concord—To promote the knowledge and practice of true religion and virtue, and the encrease of science among them and us—and generally to grant unto all Mankind such a degree of temporal prosperity as he alone knows to be best.
Given under my hand at the City of New-York the third day of October in the year of our Lord 1789.
Sarah Josepha Hale was editor of a magazine first called Ladies' Magazine and later called Ladies' Book from 1828 to 1877. It was among the most widely-known and influential magazines for women of its time. Hale wrote to Abraham Lincoln on September 28, 1863, suggesting that he set a national date for a Thankgiving holiday. From the Library of Congress, here's a PDF file of the Hale's actual letter to Lincoln, along with a typed transcript for 21st-century eyes. Here are a few sentences from Hale's letter to Lincoln:
You may have observed that, for some years past, there has been an increasing interest felt in our land to have the Thanksgiving held on the same day, in all the States; it now needs National recognition and authoritive fixation, only, to become permanently, an American custom and institution. ... For the last fifteen years I have set forth this idea in the "Lady's Book", and placed the papers before the Governors of all the States and Territories -- also I have sent these to our Ministers abroad, and our Missionaries to the heathen -- and commanders in the Navy. From the recipients I have received, uniformly the most kind approval. ... But I find there are obstacles not possible to be overcome without legislative aid -- that each State should, by statute, make it obligatory on the Governor to appoint the last Thursday of November, annually, as Thanksgiving Day; -- or, as this way would require years to be realized, it has ocurred to me that a proclamation from the President of the United States would be the best, surest and most fitting method of National appointment. I have written to my friend, Hon. Wm. H. Seward, and requested him to confer with President Lincoln on this subject ...
William Seward was Lincoln's Secretary of State. In a remarkable example of rapid government decision-making, Lincoln responded to Hale's September 28 letter by issuing a proclamation on October 3. It seems likely that Seward actually wrote the proclamation, and then Lincoln signed off. Here's the text of Lincoln's Thanksgiving proclamation, which characteristically mixed themes of thankfulness, mercy, and penitence:
October 3, 1863
By the President of the United States of America.
The year that is drawing towards its close, has been filled with the blessings of fruitful fields and healthful skies. To these bounties, which are so constantly enjoyed that we are prone to forget the source from which they come, others have been added, which are of so extraordinary a nature, that they cannot fail to penetrate and soften even the heart which is habitually insensible to the ever watchful providence of Almighty God. In the midst of a civil war of unequaled magnitude and severity, which has sometimes seemed to foreign States to invite and to provoke their aggression, peace has been preserved with all nations, order has been maintained, the laws have been respected and obeyed, and harmony has prevailed everywhere except in the theatre of military conflict; while that theatre has been greatly contracted by the advancing armies and navies of the Union. Needful diversions of wealth and of strength from the fields of peaceful industry to the national defence, have not arrested the plough, the shuttle or the ship; the axe has enlarged the borders of our settlements, and the mines, as well of iron and coal as of the precious metals, have yielded even more abundantly than heretofore. Population has steadily increased, notwithstanding the waste that has been made in the camp, the siege and the battle-field; and the country, rejoicing in the consiousness of augmented strength and vigor, is permitted to expect continuance of years with large increase of freedom. No human counsel hath devised nor hath any mortal hand worked out these great things. They are the gracious gifts of the Most High God, who, while dealing with us in anger for our sins, hath nevertheless remembered mercy. It has seemed to me fit and proper that they should be solemnly, reverently and gratefully acknowledged as with one heart and one voice by the whole American People. I do therefore invite my fellow citizens in every part of the United States, and also those who are at sea and those who are sojourning in foreign lands, to set apart and observe the last Thursday of November next, as a day of Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens. And I recommend to them that while offering up the ascriptions justly due to Him for such singular deliverances and blessings, they do also, with humble penitence for our national perverseness and disobedience, commend to His tender care all those who have become widows, orphans, mourners or sufferers in the lamentable civil strife in which we are unavoidably engaged, and fervently implore the interposition of the Almighty Hand to heal the wounds of the nation and to restore it as soon as may be consistent with the Divine purposes to the full enjoyment of peace, harmony, tranquillity and Union.
In testimony whereof, I have hereunto set my hand and caused the Seal of the United States to be affixed.
Done at the City of Washington, this Third day of October, in the year of our Lord one thousand eight hundred and sixty-three, and of the Independence of the United States the Eighty-eighth.
By the President: Abraham Lincoln
William H. Seward,
Secretary of State