1. Missouri Statehood - May 15, 1820, marked a crucial moment in Missouri history as the State of Missouri was officially admitted as a state into the Union. The Missouri Compromise of 1820 played a critical role in enabling Missouri to achieve statehood.

2. Mark Twain - One of the most famous Missourians, Mark Twain, was born on May 15, 1835, in Florida, Missouri. He is remembered mainly for his classic novels like The Adventures of Tom Sawyer, The Adventures of Huckleberry Finn, and A Connecticut Yankee in King Arthur's Court.

3. Bonnie and Clyde's Ambush - On May 15, 1934, notorious criminals Bonnie Parker and Clyde Barrow were finally taken out in an ambush orchestrated by the law enforcement officials. It took place in Sailes, Louisiana, and the Missouri state police officers played a key role in tracking down the infamous duo.

4. National Bike Month - In 1956, the League of American Bicyclists announced May as the National Bike Month to promote cycling as a healthy and sustainable mode of transportation. May 15 is celebrated as National Bike to Work Day and encourages people to cycle as an alternative to driving cars.

5. Katy Trail State Park - On May 15, 1990, Katy Trail State Park officially opened as Missouri's longest rail-trail stretching over 240 miles. It spans from Clinton to St. Charles and is unique in that it is the longest state park trail that is entirely off-road. It is named after the Missouri-Kansas-Texas railroad, which the trail was built upon.