Not to be too picky, but there was a Proclamation for a Day of Thanksgiving issued by John Hanson, while serving as President of the Continental Congress, on October 11, 1782. The Day of Thanksgiving took place on November 28, 1782.
Geo. Washington was the first president elected after the constitution was ratified. The USA had a different form of government from July 4, 1776 until December 1787.
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