1. On May 9, 1820, the first steamboat to navigate the Potomac River, the Potomac, made its maiden voyage from Georgetown to Mount Vernon. This important event marked a significant milestone in the development of transportation in the District of Columbia.

2. May 9, 1862, saw the beginning of the Civil War Battle of Spotsylvania Court House, which was fought between Union and Confederate forces in Virginia. Many soldiers from the District of Columbia participated in this historic battle.

3. On May 9, 1917, the Selective Service Act was passed by Congress, which required all men between the ages of 21 and 30 to register for the military draft. This act played an essential role in the mobilization of military resources during World War I.

4. May 9, 1930, saw the opening of the National Cathedral School, a prestigious all-girls school located in the Cathedral Heights neighborhood of Washington, D.C. The school has a long history of providing quality education to girls in the District of Columbia and beyond.

5. On May 9, 1994, the Washington Capitals won their first-ever playoff series, defeating the Pittsburgh Penguins in the first round of the NHL playoffs. This victory was a significant milestone for the Capitals, who had struggled in the playoffs in previous years.