The document also attracts interest because of the enduring mystique that surrounds Washington, with practically anything he signed, wore, or used considered valuable by collectors and institutions.
"Something this important, that brings together the fame of Washington and the emotional power of Thanksgiving, is incredibly potent," said Patrick McGrath, a books-and-manuscripts specialist at Christie's, which will take bids on Nov. 14.
Only two copies of the proclamation - handwritten by a secretary and signed by Washington - are known to exist. The other is in the Library of Congress.
This one is being sold by an owner Christie's would identify only as a private American collector.
"I do recommend and assign Thursday the 26th day of November next," Washington decreed, "to be devoted by the People of these States to the service of that great and glorious Being. . . . That we may then all unite in rendering unto him our sincere and humble thanks."
Thanks should go, he said, for "tranquillity, union, and plenty," for "the civil and religious liberty with which we are blessed," and for "the peaceable and rational manner in which we have been enabled to establish constitutions of government for our safety and happiness."
Today, it seems like Thanksgiving always has been celebrated on the fourth Thursday in November. In fact the holiday has historically wandered, proclaimed by different officials and held in different states on different days and at different times of the year - holiday chaos.
"It's a terrific opportunity for us to tell that story," said Stephanie Reyer, the Constitution Center's vice president of exhibitions. The proclamation also offers an intriguing lead-in to the center's Thanksgiving weekend exhibit, which explores everything from foods at early celebrations to the tradition of the presidential turkey pardon.
Washington, as a general and leader of the Revolutionary Army, proclaimed his first Thanksgiving in 1777 to mark the defeat of the British at Saratoga, a turning point in the war.
As president in 1789, he proclaimed the next Thanksgiving - his first under the federal government. He proclaimed a third in 1795 to note the government's defeat of a Western Pennsylvania uprising later known as the Whiskey Rebellion.
His successors embraced or ignored the holiday. Jefferson declared no Thanksgivings, concerned about the separation of church and state. He did not think the government should dedicate a day to give thanks to God.
As the country grew, Thanksgiving Day shifted among states and territories. The holiday did not settle on the calendar until the Civil War, in 1863, when Lincoln set the precedent for a national Thanksgiving Day to be celebrated on the last Thursday in November.
Future presidents followed, though Franklin D. Roosevelt tweaked the date during the Depression, moving Thanksgiving a week earlier in November in hopes of jump-starting the Christmas shopping season.
In 1941, Congress, with FDR's acquiescence, fixed Thanksgiving as the fourth Thursday of November, where it has stayed, celebrated across the U.S. from Turkey, Texas, to Cranbury, N.J.
The Washington proclamation has visited Los Angeles, Dallas, Chicago, Boston, and Washington and is due in Philadelphia Tuesday morning. It can be seen from 9:30 a.m. to 4 p.m. as part of regular admission to the Constitution Center.
One reason for the tour, Christie's McGrath said, is to publicize the auction. Another is so people can see the document, which could disappear into a private collection.
Christie bases its estimate that the proclamation is worth between $8 million and $12 million on recent sales of other Washington documents, including the president's personal copy of the Constitution, which sold last year for $9.8 million. It was bought by the Mount Vernon Ladies' Association, which owns the president's Mount Vernon estate in Virginia.
"We feel there's a very strong market," McGrath said, "for incredibly rare and important Washington material."