My friend Fred Beuttler has a wonderful tribute to Thanksgiving here. President Washington said in his original 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 and signal favors of Almighty God, especially by affording them an opportunity peaceably to establish a form of government for their safety and happiness:”
Hard to stop there. Read Fred’s post (link above) for the full text of President Washington’s proclamation.