The bustle of lunchtime foot traffic today flooded narrow streets in downtown Boston. But pedestrians took care to step over a single red flower lying at the corner of Congress and State streets.

The flower had been left on the brick monument marking the site of the Boston Massacre, which left five dead exactly 242 years ago today. On March 5, 1770, British soldiers opened fire on a crowd of colonists, killing Crispus Attucks and four others....