In 1775, Lieutenant Colonel Francis Smith, who commanded the British forces around Boston, said "Hey, it's the third Monday in April! Let's all ride out to Concord and fire the shot heard round the world!!" The importance of this event can be seen from his use of exclamation marks, a rarity in British speech.
Unbeknownst to them, however, Paul Revere and William Dawes had warned the New England Patriots, who formed a defensive line. Then, in compliance with local Stand Your Ground laws, the two sides stood blasting away at each other. Luckily muskets were not especially effective weapons.
So every year on the anniversary, Massachusetts marks this holiday with various festivities, despite the fact that in 1775, the third Monday in April fell on a Wednesday.
The festivities fall into two main categories. At the crack of dawn, people dressed as British and Colonial soldiers go out and fire muskets at each other. They use blanks, which renders their guns slightly less deadly.
This is followed by the ritual of thousands of people running halfway across the state to beat the traffic into Boston for the Red Sox game.