On Oct. 3, 1789, George Washington signed a proclamation declaring Thanksgiving a national holiday.

Now one of the original copies of this single-sided piece of paper, which includes the founding father's signature—is about to hit the auction block at Christie's.

And with the potential to fetch $8 million to $12 million, the document could set a record, said Tom Lecky, Christie's Americas head of books and manuscripts.

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"It absolutely does stand to be the most expensive American manuscript documents ever sold at auction," he told CNBC.

"Washington signed very few proclamations. It's rare enough to have a proclamation at all, but to have a proclamation about a holiday that we all hold so dear, really deserves to be featured."

Besides the document's rarity, its condition will factor into the sale price, Lecky said.

"The Library of Congress has the other only known copy, and it's not in as fine of condition. It's presumed… that 13 [copies] were sent out to the original states but only two have been located. So this is the only one in private hands and it's in superior condition," he said.

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The document will be sold at a special single-lot evening sale at Christie's New York on Nov. 14. But it won't be the first time the rare document has hit the auction block.

It was last offered up in public auction on Oct. 21, 1977, when it sold for $3,800. Christie's told CNBC the letter has changed hands privately since the 1977 public sale, but the consignor wishes to remain anonymous.

UPDATE: President George Washington's Thanksgiving Proclamation did not find a buyer at the auction, but Christie's told CNBC, "Given its historical significance, it may sell privately."

—By CNBC's Erica Emmich. Follow her on Twitter and Instagram: @MegaMoneyGal.

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