Here’s one of ten fun facts about Thanksgiving from Erin at the OUP Blog:
5. A few months after the North’s military victories at Gettysburg and Vicksburg in the summer of 1863, Lincoln declared the last Thursday in November a national day of Thanksgiving. Every president since has proclaimed Thanksgiving Day a national holiday. However, the first president to issue a Thanksgiving proclamation was George Washington, who did so at the direction of Congress on 3 October 1789. The few presidents who subsequently issued Thanksgiving proclamations commemorated particular events, such as President James Madison’s proclamation of celebration at the end of the War of 1812. Few presidents issued Thanksgiving proclamations thereafter.
The rest are at the link!