1. The first ever bank robbery in the United States occurred on March 20, 1798, in Philadelphia, Pennsylvania. The robbed bank was the Bank of Pennsylvania, when robbers stole $162,821 from the vault in broad daylight.

2. On March 20, 1813, the Pennsylvania Legislature approved the creation of Allegheny County from part of Washington and Westmoreland counties. Allegheny County is the second most populous county in Pennsylvania, and its county seat is Pittsburgh.

3. In 1844, the Pennsylvania General Assembly passed a law prohibiting the employment of children under the age of 12 in factories or mines. This law, known as the Child Labor Act, was one of the earliest laws to protect children's welfare in the United States.

4. On March 20, 1902, the Pennsylvania State Capitol Building in Harrisburg was completed after almost 11 years of construction. The building, designed in the Beaux-Arts style, is one of the most majestic buildings in Pennsylvania and houses the offices of the governor, lieutenant governor, and other state officials.

5. In 1979, Three Mile Island nuclear power plant suffered a partial meltdown on March 28, 1979. However, on March 20, 1979, was the day when a mechanical failure began the chain of events leading to the meltdown. This accident became the worst accident in the history of American commercial nuclear power, and it raised awareness about nuclear power and its risks.