Today, it's mainly a day for free parking downtown and for Boston municipal workers and school kids to get a day off. But back in the day, it honored the first major victory of the Revolution - the day the British fled the capital of the colony where the fighting had begun.
For months, Boston had been a town under seige: After Lexington and Concord, the Redcoats had retreated to Boston, then a hilly neck easily defended by land and sea. Unless the colonials could somehow get cannons up on Dorchester Heights - another peninsula to its south with a clear shot at Boston. Which they did - thanks to Henry Knox and his oxen (with wagon wheels wrapped with straw to dampen their noise).
Gen. Howe spotted the new cannons and planned to take the hills - but then it rained, hard enough to prevent an assault, but not hard enough to keep Washington's men from reinforcing their armaments. When the clouds cleared, Howe realized he had no chance and decided to avoid ignominious surrender by fleeing - after reaching an agreement with Washington to not burn down Boston in exchange for being allowed to leave unmolested.
J.L. Bell posts a letter from Col. Jedidiah Huntington on watching the British troops leave:
This morning we had the Agreeable Sight of a number of ships leaving the Town of Boston with a large number of Boats full of Soldiers, about ten of Clock several Lads came to our out Centries and informed us that the Troops had intirely left the Town and that the Selectmen were coming out to see us.