American Thanksgiving is the fourth Thursday in November. Countless institutions depend on this date being predictable year in and year out: football teams planning their "Turkey Bowl" games, schools setting their vacation schedules, department stores deciding when to put up their Christmas decorations.
But the Thanksgiving date wasn't always so reliable. For decades, the president got to decide when the holiday fell each year. They tended to follow the example of Abraham Lincoln, who in 1863 set Thanksgiving on the final Thursday of November—until Franklin D. Roosevelt went rogue. In 1939 there were five Thursdays in November, so according to Lincoln's example, Thanksgiving would have been on the last day of the month. Roosevelt worried that such a late Thanksgiving would mean that Christmas shopping season would be too short, which would be hard on merchants already hurting from the Great Depression. So he declared that Thanksgiving that year would fall on the second-to-last Thursday, November 23rd.
Though the change was intended to help a struggling nation, it ended up dividing the country. Twenty-two states refused to go along with Roosevelt's decision and instead celebrated Thanksgiving on the 30th. Almost all the other states sided with the president and feasted the week before. (Texas, Mississippi, and Colorado couldn't make up their minds and declared both the 23rd and the 30th holidays.)