Henry Knox brings the artillery though tough terrain to Boston (From the National Archives)
Evacuation day celebrates March 17, 1776, the day the British Army left Boston after a prolonged siege. Here's some background on the days leading up to it.
The Seige of Boston
Wikipedia outlines what led up to the Colonials isolating the city of Boston with the British in control:
The Siege of Boston (April 19, 1775 – March 17, 1776) was the opening phase of the American Revolutionary War, in which New England militiamen—who later became part of the Continental Army—surrounded the town of Boston, Massachusetts, to prevent movement by the British Army garrisoned within. After eleven months of siege, the American colonists, led by George Washington, forced the British to withdraw by sea.
The siege began on April 19 after the Battles of Lexington and Concord, when the militia from many Massachusetts communities surrounded Boston and blocked land access to the then-peninsular town, limiting British resupply to naval operations. The Continental Congress chose to adopt the militia and form the Continental Army, and unanimously elected George Washington as its Commander in Chief. In June 1775, the British seized Bunker and Breed's Hills, but the casualties they suffered were heavy and their gains were insufficient to break the siege. For the rest of the siege, there was little action other than occasional raids, minor skirmishes, and sniper fire. Both sides had to deal with resource supply and personnel issues over the course of the siege, and engaged in naval operations in the contest for resources.
In November 1775, Washington sent a 25 year-old bookseller-turned-soldier named Henry Knox to bring heavy artillery that had been captured at Fort Ticonderoga to Boston. In a technically complex and demanding operation, Knox brought many cannons to the Boston area in January 1776.
General Washington Places Artillery on Dorcester Heights
Once Washington had the artillery from Ticonderoga, he wasted no time in deploying it.
Washington first placed some of the heavy cannons from Ticonderoga at Lechmere's Point and Cobble Hill in Cambridge, and on Lamb's Dam in Roxbury. As a diversion against the planned move on the Dorchester Heights, he ordered these batteries to open fire on the town on the night of March 2, which fire the British returned, without significant casualties on either side. These cannonades were repeated on the night of March 3, while preparations for the taking of the heights continued.
On the night of March 4, 1776, the batteries opened fire again, but this time the fire was accompanied by action. General John Thomas and about 2,000 troops quietly marched to the top of Dorchester Heights, hauling entrenching tools and cannon placements. Hay bales were placed between the path taken by the troops and the harbor in order to muffle the sounds of the activity. Throughout the night, these troops and their relief labored at hauling cannon and building earthworks overlooking the town and the harbor. General Washington was present to provide moral support and encouragement, reminding them that March 5 was the sixth anniversary of the Boston Massacre.By 4 am, they had constructed fortifications that were proof against small arms and grapeshot. Work continued on the positions, with troops cutting down trees and constructing abbatis to impede any British assault on the works.The outside of the works also included rock-filled barrels that, while appearing to be a part of the defensive structure, could be rolled down the hill at attacking troops.
British General Howe the next day was quoted as saying, "The rebels have done more in one night than my whole army would have done in a month."
In the map below you can see the strategic advantage of having the artillery on the heights would bring. It is marked with a 4 on the map. There was only one way in and out of Boston by land at that time. Over Boston Neck.
The British Leave Boston
The British wanted out but were worried that a Colonial attack would hurt them badly. They made a deal:
Seeing the American fortifications on the heights in the morning, Howe initially made plans for assaulting the position. This was prevented by a snowstorm late in the day. Unable to attack, Howe reconsidered his plan and elected to withdraw rather than have a repeat of Bunker Hill. On March 8, Washington received word that the British intended to evacuate and would not burn the city if allowed to leave unmolested. Though he did not formally respond, Washington agreed to the terms and British began embarking along with numerous Boston Loyalists. On March 17, the British departed for Halifax, Nova Scotia and American forces entered the city. Having been taken after an eleven-month siege, Boston remained in American hands for the remainder of the war.
Boston's Yearly Celebration
The City of Boston has declared Evacuation Day a holiday. Yes… a holiday. It just so happens that it is also St. Patrick's day. Today, the focus of the day's activities is much more around St. Patrick's day than it is celebrating evacuation day. But years ago, the focus was on that great Colonial victory. Here's some pictures form the 1937 parade with the military on full display.