1. Johnstown Flood: On May 31, 1889, the infamous Johnstown Flood occurred in Pennsylvania. A massive dam break caused water to surge through the town, killing over 2,200 people and causing immense destruction.

2. Statehood: May 31, 1790, marked the date that Pennsylvania became the second state to ratify the US Constitution, after Delaware. This was a significant moment in the early history of the United States.

3. Fort Pitt Treaty: On May 31, 1779, the Fort Pitt Treaty was signed which was a peace treaty between the United States and several Native American tribes. The treaty marked a turning point in relations between the two groups and facilitated the settlement of western Pennsylvania.

4. Pennsylvania Railroad: May 31, 1854, was the day that the Pennsylvania Railroad was chartered. At the time, it was the largest railroad company in the world and played a significant role in the development of the state and the country.

5. Memorial Day: While not specific to Pennsylvania, May 31 is often noted as the traditional date for Memorial Day, a national holiday honoring US military personnel who died in the line of duty. This observance has its roots in the post-Civil War era and remains an important day of remembrance in Pennsylvania and throughout the US.