From Smithsonian:
Colonial Boston’s secret patriot network crackled with the news. Regiments of British troops were on the move, bound for points north to secure military supplies from the rebels. Paul Revere mounted his horse and began a feverish gallop to warn the colonists that the British were coming. Except this ride preceded Revere’s famous “midnight ride” by more than four months. On December 13, 1774, the Boston silversmith made a midday gallop north to Portsmouth in the province of New Hampshire, and some people—especially Granite Staters—consider that, and not his trip west to Lexington on April 18, 1775, as the true starting point of the war for independence.
With talk of revolution swirling around Boston in the final days of 1774, Revere’s patriot underground learned that King George III had issued a proclamation that prohibited the export of arms or ammunition to America and ordered colonial authorities to secure the Crown’s weaponry. One particularly vulnerable location was Fort William and Mary, a derelict garrison at the mouth of Portsmouth Harbor with a large supply of munitions guarded by a mere six soldiers. When Boston’s Committee of Correspondence, a local group of citizens opposed to British rule, received intelligence that British General Thomas Gage had secretly dispatched two regiments by sea to secure the New Hampshire fort—a report that was actually erroneous—they sent Revere to alert their counterparts in New Hampshire’s provincial capital. Just six days after the birth of his son Joshua, Revere embarked on a treacherous wintry journey over 55 miles of frozen, rutted roads. A frigid west wind stung his cheeks, and both rider and steed endured a constant pounding on the unforgiving roadway.
More here.