But that wasn’t yet the Thanksgiving, it was just a Thanksgiving.
For about as long as there were settlers in America, there were days set aside for “Thanksgiving.” Of course, the Plymouth settlers did so, probably sometime in the 1621 autumn, although the exact date is unknown. Colonial governments had numerous Thanksgivings, especially during the Revolution to honor victories.
The first known “national” observance of the tradition was Dec. 18, 1777. Washington declared a few Thanksgivings in his time, with varying dates, as did subsequent presidents.
The practice finally became standardized in 1863, when President Abraham Lincoln made the last Thursday of every November a national Thanksgiving. This initial celebration largely came from celebrating the Union’s Gettysburg victory.
That practice stuck until 1939, when President Franklin D. Roosevelt changed Thanksgiving to the second-to-last Thursday, giving businesses a larger Christmas shopping window amid the end of the Great Depression. That year there were five Thursdays in the month. An uproar ensued that this commercialized the holiday, and so many states stuck to celebrating on the last Thursday.
Then in 1941, Roosevelt and Congress made another, more final update, calling for Thanksgiving to be observed on the fourth Thursday of November.
For more on this subject ...
National Geographic explored the early Thanksgivings.
Mount Vernon, the museum of Washington’s former home, wrote about his involvement.
The Farmer’s Almanac gave a more theoretical history about why Thursday might have been the initial day of choice.
Businesses actually asked FDR to change the date earlier in his presidency in 1933. Here’s that letter.
NPR detailed the FDR aspect.
The National Archives’ entry for FDR’s 1941 date change.