It is likely little known that Boston has two celebrations on March 17th each year... on the one hand, they celebrate the Irish... and the other they celebrate the Evacuation of the British... they call it Evacuation Day. In 1775, the British had blockaded the Boston Harbor... it was a troublesome time, around the American Revolution.
There were the British in the harbor trying to starve the Americans into submission. They were having nothing of it, and plotted to move canon into place on the hills overlooking the harbor... that was laden with the British blockade of ships. While this had all transpired over many battles, and eleven months... it was on March 17th, 1776 that the British realized that the Americans had, using subterfuge, successfully moved canon from the captured Fort Ticonderoga into place, and they were sitting ducks in the harbor. The British, on the first tide and favorable winds, retreated to Halifax... just another in many ties Boston has with Halifax, Nova Scotia.
So, the Irish celebrate not only St. Paddy's Day... but a veritable expulsion of the British that they appropriately name "Evacuation Day".