In the past, Patriots' Day was on April 19. This was done in honor of Lexington and Concord. These were the first battles of the American Revolutionary War that were fought on that date in 1775. Since 1969, however, the holiday has been on the third Monday in April.
The Boston Marathon is run on this day every year. The Boston Marathon bombings happened on this day in 2013. The Boston Red Sox have played a home game at Fenway Park on this day every year since 1960.