From the mid-nineteenth century to the early twentieth century, in a custom started by US President Abraham Lincoln, the president would declare a "day of thanks giving" on the last Thursday in November. This could either fall on the fourth or fifth Thursday in the month.
But in 1939, a funny thing happened - the last Thursday happened to be the last day in November. Retailers, worried about the shortened holiday shopping season, petitioned President Franklin Delano Roosevelt to declare the holiday a week earlier - which he did.
For the next three years, Thanksgiving was known derisively as "Franksgiving" and celebrated on different days in different parts of the country.
Finally, at the end of 1941, a joint resolution from Congress cleared up the matter. From then on, Thanksgiving would be celebrated on the fourth Thursday in November - guaranteeing an extra week of shopping before Christmas.