April 18th is the anniversary of the 2 lanterns. See below, link at bottom.
Founded in 1722, the Old North Church, officially known as Christ Church in the City of Boston, is Boston’s oldest surviving church building and one of its most popular historic sites.
Inside the Old North Church | Historic Boston
Looking towards the Paul Revere statue and the rear of the historic Old North Church in Boston, Massachusetts.
It is most famous for the role it played in the American Revolution on the evening of April 18, 1775. That night, church sexton Robert Newman and vestryman Capt. John Pulling, Jr. climbed the steeple and lit two lanterns to signal that the British were marching to Lexington and Concord by sea (across the Charles River), rather than by land. This served as the spark to Paul Revere’s famous midnight ride (which we all know is more legend than fact, but let’s save that for another post), and the start of the American Revolution.
Site:
https://newengland.com/today/travel/massachusetts/old-north-church-2/