Three Flags Day was when on 9 and 10 March 1804 Spain gave Louisiana to France, who then gave the entire territory to the United States. What became known as the Louisiana Purchase took place in St. Louis. The Spanish flag was taken down on 9 March, the French flag flew for one day and then the US flag was raised. Very symbolic.
Above is the site of the ceremony to mark the near doubling of United States territory. It was certainly an incredible piece of negotiation, the intention had been to acquire New Orleans from the French for around $10 million and instead they got 530 million acres for $15 million. Bargain.