1. On December 19, 1777, George Washington and his army arrived at Valley Forge in Pennsylvania after a long and difficult march from their previous encampment in Whitemarsh, Pennsylvania. Massachusetts soldiers played a crucial role in the Continental Army during the Revolutionary War, and many of them were among the soldiers who suffered through the harsh winter at Valley Forge.

2. On December 19, 1843, Boston's Tremont Street Subway opened to the public, making it the first subway tunnel in the United States. The subway was a major engineering feat that relied on new innovations in tunneling and ventilation. Today, the subway system is an integral part of Boston's public transportation network.

3. On December 19, 1960, the Boston Celtics defeated the Detroit Pistons in a dramatic double-overtime game. The win set the stage for the Celtics to go on and win their third NBA championship in a row, solidifying their status as one of the greatest basketball dynasties of all time. The Celtics have continued to be a beloved Boston sports team, winning a total of 17 NBA championships to date.

4. December 19 is also the birthday of Boston-born actor Tim Reid, best known for his roles in the TV shows "WKRP in Cincinnati" and "Sister, Sister." Reid began his career as a stand-up comedian, but quickly transitioned to acting and has since appeared in numerous TV shows, movies, and stage productions.

5. Finally, on December 19, 2017, a new law went into effect in Massachusetts legalizing the recreational use of marijuana. Massachusetts was the first state on the East Coast to legalize recreational marijuana, and the move has been seen as a major milestone in the movement to end the prohibition of cannabis nationwide.