is Established as
Thanksgiving had been celebrated as a special holiday in 1789 by George Washington.
(That proclamation is also preserved at the Karpeles Manuscript Library)
Thanksgiving was also celebrated by Presidential Proclamations several times after that including once by Abraham Lincoln in 1863, but Lincoln had no intention of making it permanent. In fact, in 1864, he had to be petitioned to again celebrate the day.
The year 1864 marks the beginning of the practice of a national celebration each year, fixing the holiday on the fourth Thursday (later it came to be regularly the last Thursday) in November.
"My proclamation of that date being the setting aside by presidential etiquette of the last Thursday of November as "A Day of Thanksgiving and Praise to Almighty God".