The following quotes are taken from several Presidential Thanksgiving Proclamations:

“It is eminently fitting that once a year our people should set apart a day for praise and thanksgiving to the Giver of Good, and at the same time that they express their thankfulness for the abundant mercies received should manfully acknowledge their shortcomings and pledge themselves solemnly and in good faith to strive to overcome them.”
President Theodore Roosevelt, Nov. 2, 1905

“On [Thanksgiving] let us gather in sanctuaries dedicated to worship and in homes blessed by family affection to express our gratitude for the glorious gifts of God; and let us earnestly and humbly pray that He will continue to guide and sustain us in the great unfinished tasks of achieving peace, justice, and understanding among all men and nations and of ending misery and suffering wherever they exist.”
President John F. Kennedy, Nov. 4, 1963

“For the harvest that has sustained us and, in its fullness, brought succor to other peoples; for the bounty of our soil, which has produced the sinews of war for the protection of our liberties; and for a multitude of private blessings, known only in our hearts, we should give united thanks to God.¬† To the end that we may bear more earnest witness to our gratitude to Almighty God, I suggest a nationwide reading of the Holy Scriptures during the period from Thanksgiving Day to Christmas.”
President Franklin D. Roosevelt, Nov. 1, 1944

“Today let us take heart from the noble example of our first President. Let us pause from our many activities to give thanks to almighty God for our bountiful harvests and abundant freedoms. Let us call upon Him for continued guidance and assistance in all our endeavors.”
President Ronald Reagan, Oct. 13, 1986Thanksgiving Day, 1986

“America’s oldest tradition, Thanksgiving is also a reaffirmation of our most deeply held values; a public recognition that, in the words of Thomas Jefferson, ‘God who gave us life gave us liberty.’ In gratitude for God’s gift of freedom and ‘for all the great and various favors which he hath been pleased to confer upon us,’ ¬†George Washington made Thanksgiving his first proclamation for the new Nation, and it is one we are privileged to renew each year.”

Read all Presidential Thanksgiving Proclamations at pilgrimhall.org