"As no truth is more clearly taught in the Volume of Inspiration, nor any more fully demonstrated by the experience of all ages, than that a deep sense and a due acknowledgment of the governing providence of a Supreme Being and of the accountableness of men to Him as the searcher of hearts and righteous distributer of rewards and punishments are conducive equally to the happiness and rectitude of individuals and to the well-being of communities...For these reasons I have thought proper to recommend, and I do hereby recommend accordingly, that Thursday, the 25th day of April next, be observed throughout the United States of America as a day of solemn humiliation, fasting, and prayer; that the citizens on that day abstain as far as may be from their secular occupations, devote the time to the sacred duties of religion in public and in private; that they call to mind our numerous offenses against the Most High God, confess them before Him with the sincerest penitence, implore His pardoning mercy, through the Great Mediator and Redeemer, for our past transgressions, and that through the grace of His Holy Spirit we may be disposed and enabled to yield a more suitable obedience to His righteous requisitions in time to come; that He would interpose to arrest the progress of that impiety and licentiousness in principle and practice so offensive to Himself and so ruinous to mankind; that He would make us deeply sensible that 'righteousness exalteth a nation, but sin is a reproach to any people'...And I do also recommend that with these acts of humiliation, penitence, and prayer, 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." (John Adams)
"In this year of liberation, which has seen so many millions freed from tyrannical rule, it is fitting that we give thanks with special fervor to our Heavenly Father for the mercies we have received individually and as a nation and for the blessings He has restored, through the victories of our arms and those of our allies, to His children in other lands....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. Let every man of every creed go to his own version of the Scriptures for a renewed and strengthening contact with those eternal truths and majestic principles which have inspired such measure of true greatness as this nation has achieved." (Franklin D. Roosevelt)
"Although the time and date of the first American thanksgiving observance may be uncertain, there is no question but that this treasured custom derives from our Judeo-Christian heritage. 'Unto Thee, O God, do we give thanks,' the Psalmist sang, praising God not only for the 'wondrous works' of His creation, but for loving guidance and deliverance from dangers....Let us thank God for our families, friends, and neighbors, and for the joy of this very festival we celebrate in His name." (Ronald Reagan)
For a large collection of thanksgiving proclamations from the Continental Congress and American presidents down to our own time, see here. See, also, here.