1. On April 20, 1908, the cornerstone of the Daughters of the American Revolution (DAR) Constitution Hall was laid in Washington D.C. A national headquarters for the organization was needed and a fundraising campaign helped secure the land and construction costs. Today, the hall serves as a venue for concerts, theater productions, and other cultural events.

2. In 1971, April 20 became an important date for environmentalists in D.C. as it marked the first Earth Day celebration. The event was organized by Senator Gaylord Nelson and aimed to raise awareness about the impact of pollution on the planet. Over 20 million people across the country participated in various environmental activities.

3. On April 20, 1999, the Columbine High School massacre shocked the nation and the District of Columbia. The tragedy, which took place in Colorado, left 15 dead, including the two teenage attackers. The event led to renewed debates about gun control laws and school safety.

4. In 2002, April 20 became significant in D.C. for a different reason as it marked the day that the city began allowing medical marijuana use. The law, called the Medical Marijuana Program Amendment Act, allowed patients with certain conditions to use marijuana for medicinal purposes with a doctor's recommendation.

5. In 2010, on April 20, the Deepwater Horizon oil rig exploded in the Gulf of Mexico, leading to one of the worst environmental disasters in U.S. history. The oil spill, which lasted for months, affected wildlife, fishing industries, and tourism along the Gulf Coast. The incident prompted calls for better safety regulations and environmental protection measures.