1. In 1938, the groundbreaking ceremony for the Allegheny County Courthouse was held. The courthouse is located in downtown Pittsburgh and is renowned for its distinctive architecture, which features Gothic Revival and Romanesque elements.

2. On April 18, 1955, a polio vaccine was declared safe and effective during a landmark trial conducted at the University of Pittsburgh. The vaccine was developed by Dr. Jonas Salk and proved to be a critical breakthrough in the fight against polio, which had been a major public health concern in the United States for decades.

3. In 1775, a group of American soldiers led by Col. Benedict Arnold seized a British supply depot in Ticonderoga, New York. Among the captured supplies were dozens of cannons, which would later be used by the Continental Army during the siege of Boston. Many of these cannons were transported across Pennsylvania on their way to Boston.

4. On April 18, 1899, the Pennsylvania Railroad began electrifying its tracks between Philadelphia and Paoli, Pennsylvania. The electrification project was the first of its kind in the United States and marked a major turning point in the history of railroad transportation.

5. In 1936, the World Series of Professional Football was held in Philadelphia. The championship game featured the Green Bay Packers and the Boston Redskins, with the Packers emerging victorious by a score of 21 to 6. The game was played at the historic Franklin Field, which was also the site of the first Penn Relays event in 1895.