1. Battle of Brandywine: On September 17, 1777, the Battle of Brandywine took place in Pennsylvania during the American Revolution. The battle saw the forces of British General Sir William Howe defeat General George Washington's Continental Army, resulting in the British occupation of Philadelphia.

2. Constitution Day: September 17 is also observed as Constitution Day in the United States. This day commemorates the signing of the U.S. Constitution in Philadelphia in 1787.

3. Flight 93 Memorial: On September 17, 2001, the Flight 93 National Memorial was dedicated in Somerset County, Pennsylvania. The memorial honors the victims of United Airlines Flight 93, which crashed in a field during the 9/11 terrorist attacks.

4. Pittsburgh Railway Riot: In 1877, Pittsburgh saw one of the deadliest railway strikes in U.S. history. On September 17 of that year, the Pittsburgh Railway Riot erupted, resulting in the deaths of over 20 people.

5. Lancaster County Prison Riot: On September 17, 1979, a riot broke out at the Lancaster County Prison in Lancaster, Pennsylvania. The riot lasted for two days and saw over 1,000 inmates take control of the facility, resulting in the deaths of two inmates and injuries to several others.