In Lincoln’s original declaration of the last Thursday in November as a National holiday, he designated it as a “Day of Thanksgiving and Praise.”
In 1939, FDR changed Thanksgiving to the next to last Thursday. Roosevelt hoped that the lengthened Christmas shopping season would lift the nation out of it’s economic slump. The declaration was not compulsory and only about half the states went along with his recommendation.
In 1941, Congress voted that Thanksgiving would occur annually on the fourth Thursday in November.