annually on April 19th (1894 to 1968)
the third Monday in April (since 1969)
Patriots' Day is a holiday commemorating the battles of Lexington and Concord, which took place on April 19, 1775. The shots fired at these two cities on the outskirts of Boston were the first shots of the American Revolution. Following the war, Lexington Day and Concord Day began being celebrated in their respective cities. The cities later petitioned Massachusetts Governor Frederic Greenhalge to create a state commemoration. He created Patriots' Day, which replaced Fast Day. It was first celebrated on April 19, 1894. The new holiday also originally commemorated the anniversary of the Baltimore riot of 1861, where some of the first bloodshed of the Civil War took place, and four members of the Massachusetts militia died. In 1938, the Massachusetts legislature passed a bill making the battles of Lexington and Concord the sole focus of the day.
Until 1969, the holiday was celebrated on the actual anniversary of the battles, but it since has been observed on the third Monday of April in most states that officially commemorate it. As of 2018, it is a state holiday in Maine and Massachusetts. Schools in Wisconsin observe it on April 19 (if it's a weekday) by having students learn about the battles and their importance to American history. Those in Florida are encouraged to celebrate the day, although it isn't an official holiday there. Connecticut began officially observing the day in 2018.
Reenactments of the battles are held at Lexington Green in Lexington and at the Old North Bridge in Concord. Paul Revere, who is known for his "midnight ride" on the night before the battle is celebrated on the day. In fact, reenactments of his ride, as well as that of William Dawes are held. Lectures and concerts are also held, and the Boston Marathon is part of the celebration.
How to Observe
Celebrate the day by visiting Minute Man National Historical Park, where reenactments take place. There are many other events held in the Boston area on the day that you could attend. If you can't make it to any commemorations of the day, you could learn more about the battles on your own.