1. The signing of the Organic Act of 1801
On December 20, 1801, President Thomas Jefferson signed the Organic Act of 1801, which established the District of Columbia as the permanent capital of the United States.

2. The opening of the Smithsonian Institution Building
On December 20, 1855, the Smithsonian Institution Building, also known as the Castle, was officially opened to the public. It is now the headquarters of the Smithsonian Institution.

3. The assassination of John F. Kennedy Jr.
On July 16, 1999, John F. Kennedy Jr. died in a plane crash off the coast of Martha's Vineyard. He was flying to a wedding in Hyannis Port, Massachusetts with his wife, Carolyn Bessette, and her sister, Lauren Bessette.

4. The Metro system opens
On December 20, 1976, the Washington Metro, a rapid transit system serving the Washington metropolitan area, began operation. It currently consists of six lines and 91 stations.

5. A notable murder trial verdict
On December 20, 2010, a Washington, D.C. jury found Ingmar Guandique guilty of the 2001 murder of Chandra Levy, a government intern whose case had received national attention due to her relationship with then-Congressman Gary Condit.