March 10 is known as Three Flags Day in St. Louis, Missouri. The city was part of the Louisiana Purchase in 1803, which expanded the US into what later became all or part of 15 states. But see, France had just gained most of that land from Spain mere weeks before the deal with the United States, and some cities remained under Spanish administration for quite some time. A treaty ceremony was held New Orleans, but other areas didn't get the news until later. So they planned another ceremony in St. Louis for March 9, 1804.
But there was a small problem: St. Louis was one of the cities the Spanish hadn’t gotten around to giving up. Authorities soon realized that, in order to actualize the territory’s new identity, “two treaties must be put into effect at one time,” writes historian Walter Barlow Stevens in St. Louis, The Fourth City. First, the land had to be be transferred from Spain to France. Then, it had to be be transferred again, from France to the United States.
St. Louis residents were so happy to be suddenly French that the powers-that-be hesitated to tell them they were actually American. So they delayed the second ceremony until the next day! Read about the handover of St. Louis at Atlas Obscura.