At some point in time, Boston decided to have a holiday for the Irish. St. Patrick's Day was the natural choice, but it is, after all, a Catholic observance. How could this be made an official holiday? By happy coincidence, it is also the day in history in 1776 that Washington mounted cannon on Dorchester Heights (in South Boston, actually), forcing British General Howe to evacuate the city.
The artillery pieces were captured from Ft. Ticonderoga (in New York) by Ethan Allen and Benedict Arnold and the Green Mountain Boys on May 10, 1775, who made a surprise nighttime raid. The British commander was awakened at sabre-point by Allen who demanded his surrender "in the name of the Continental Congress and the Great Jehovah!" The artillery was hauled across the colony on ox-pulled sledges the following November by a 25-year-old bookseller named Henry Knox (for whom Ft. Knox in Kentucky is named) and delivered to Washington, thus forcing the British to evacuate the city.
St. Patrick's Day parades began in 1875. The "Evacuation Day" (St. Patrick's Day) as an official holiday was established in 1901, but is official only in Suffolk County: Boston, and the adjacent municipalities of Chelsea, Revere, and Winthrop.