Now, here is an interesting question. Why did the same Congress which allegedly passed a Constitutional amendment which totally separates church and state call for a day of thanksgiving?
It was done at the end of the legislative session. It was considered appropriate by the legislative body for the citizens of the new United States to set aside a day and thank God for the blessings He had given them in the new country. And they passed a resolution to that effect.
This particular action is important because it shows that the First Congress didn't wish merely to contain religion amongst themselves, as in the case with Congressional prayer and chaplains, but here they encourage the practice of religion amongst the citizens of the United States. Further, they put a stamp of approval on that religious action by passing a resolution to that effect.
So, doesn't this stand rather oddly against the notions of some who claim that our country's founders wanted to separate religion completely from government? Here we have the first Congress performing three distinct acts which bring religion into the public arena: hiring chaplains, Congressional prayers, and declaring a day of thanksgiving to Almighty God.
But alas, there is more.
 See Gorton Carruth's Encyclopedia of American Facts and Dates
(10th ed., p. 107), where he puts the date of the proclamation at November