1. On December 20, 1783, General George Washington resigned his commission as commander-in-chief of the Continental Army to the Congress of the Confederation in the Maryland State House in Annapolis, Maryland. This event marked the end of the Revolutionary War and the beginning of Washington's retirement from public life.

2. In 1843, the first Pennsylvania railroad opened for public use on December 20th. The Philadelphia and Reading Railroad Company charged a fare of 5 cents per mile, which was considered expensive at the time. However, it was a major milestone in the development of the state's transportation infrastructure and helped to boost the economy.

3. On December 20, 1860, the state of South Carolina became the first state to secede from the United States. This was a turning point in the lead up to the Civil War, as several other southern states followed suit and formed the Confederate States of America.

4. The first shipment of Coca-Cola to Pennsylvania arrived on December 20, 1899. The popular beverage was introduced to the state by the Coca-Cola Bottling Company of Philadelphia, which began distributing the drink to local stores and restaurants. Today, Coca-Cola is still one of the most widely consumed drinks in the state.

5. On December 20, 1999, the Pennsylvania Senate voted to acquit Justice Rolf Larsen of impeachment charges, ending a months-long political drama that had engulfed the state's judiciary. Larsen had been accused of using his office to influence a case involving a political donor, but was ultimately cleared of all charges. The case had significant implications for the state's legal system, as it raised questions about the independence of the judiciary and the role of political influence in the courts.