“I recommend to all citizens to meet in their respective places of worship on Thursday the 24th day of November next, there to give thanks for the bounty of God during the year about to close and to supplicate for its continuance hereafter.”
from President Ulysses S. Grant’s Thanksgiving proclamation, 1870

Read all of the presidential Thanksgiving proclamations at pilgrimhallmuseum.org/thanksgiving_proclamations.htm (and read excerpts from other presidents below):

“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.”
George Washington in his 1789 Thanksgiving day proclamation

“And I do also recommend that with these acts of humiliation, penitence, and prayer [to God], fervent thanksgiving to the Author of All Good be united for the countless favors which He is still continuing to the people of the United States, and which render their condition as a nation eminently happy when compared with the lot of others.”
President John Adams, 1799

“No people ought to feel greater obligations to celebrate the goodness of the Great Disposer of Events of the Destiny of Nations than the people of the United States. His kind providence originally conducted them to one of the best portions of the dwelling place allotted for the great family of the human race. He protected and cherished them under all the difficulties and trials to which they were exposed in their early days. … I now recommend that [a day] be set apart as a day on which the people of every religious denomination may in their solemn assembles unite their hearts and their voices in a freewill offering to their Heavenly Benefactor of their homage of thanksgiving and of their songs of praise.”
President James Madison, 1815

“I, Abraham Lincoln, President of the United States, do hereby appoint and set apart the last Thursday in November next as a day which I desire to be observed by all my fellow-citizens, wherever they may then be, as a day of thanksgiving and praise to Almighty God, the beneficent Creator and Ruler of the Universe. And I do further recommend to my fellow-citizens aforesaid that on that occasion they do reverently humble themselves in the dust and from thence offer up penitent and fervent prayers and supplications to the Great Disposer of Events for a return of the inestimable blessings of peace, union, and harmony throughout the land which it has pleased Him to assign as a dwelling place for ourselves and for our posterity throughout all generations.”
President Abraham Lincoln, 1864

“I earnestly recommend that, withdrawing themselves from secular cares and labors, the people of the United States do meet together on that day in their respective places of worship, there to give thanks and praise to Almighty God for His mercies and to devoutly beseech their continuance.”
President Dwight D. Eisenhower, 1958

“As was written in the first Thanksgiving Proclamation 120 years ago, “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.” God has blessed America and her people, and it is appropriate we recognize this bounty.”
President Ronald Reagan, 1982

Read all of the presidential Thanksgiving proclamations at: pilgrimhallmuseum.org/thanksgiving_proclamations.htm.