1. On April 20, 1777, Philadelphia served as the temporary capital of the United States during the Revolutionary War. The Continental Congress moved to the city for safety reasons following British attacks on the former capital, New York City.

2. In 1861, the Pennsylvania Supreme Court began hearing arguments in the case of William McCardle. McCardle, a newspaper editor who had openly opposed the Union during the Civil War, was arrested and charged with treason. The case became a landmark in the balance of power between the federal government and the states.

3. On April 20, 1914, a coal mine explosion in Eccles, West Virginia, claimed the lives of 183 men and boys, including 11 from Pennsylvania. The tragedy highlighted the dangers of coal mining and led to improved safety regulations for miners.

4. In 1939, the Pennsylvania Turnpike opened to motorists, becoming the first long-distance highway with limited access in the United States. The turnpike revolutionized travel in Pennsylvania and set the standard for highways across the country.

5. In 1999, two high school students in Littleton, Colorado, committed a mass shooting at Columbine High School, killing 13 people and injuring 24 others. The tragedy sparked national debate on gun control and school safety, and led to changes in how schools prepare for and respond to potential threats.