1. On April 21, 1789, John Harris Jr. died in Harrisburg, Pennsylvania. Harris was one of the earliest settlers in the area, and his family played a significant role in the founding and development of the city of Harrisburg.

2. April 21, 1898, marked the beginning of the Spanish-American War, which would last until August of that year. Pennsylvania played a significant role in the war effort, sending thousands of soldiers to fight and contributing much-needed resources like coal and steel.

3. On April 21, 1906, the Allegheny Observatory in Pittsburgh was formally dedicated. The observatory, which was founded in 1859, remains a significant center for astronomical research to this day.

4. April 21, 1934, saw the dedication of Philadelphia City Hall. The massive granite building, which took 30 years to complete, is the largest municipal building in the country and a National Historic Landmark.

5. Finally, April 21, 1961, saw the first human spaceflight, as Soviet cosmonaut Yuri Gagarin orbited the Earth. Although Gagarin did not visit Pennsylvania on his historic flight, the state's many contributions to the American space program, including the creation of the Lunar Module that would take astronauts to the moon, make this an important event in Pennsylvania's history as well.