Elsewhen, George Washington kicked this whole thing off with his Thanksgiving Proclamation of 1789. Here's the opening bit.
By the President of the United States of America, a Proclamation.Read, as they say, the whole thing. It's worth a little such reflection once in a while.
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:”