From History.com – On this day in history – April 18, 1775 – 

… British troops march out of Boston on a mission to confiscate the American arsenal at Concord and to capture Patriot leaders Samuel Adams and John Hancock, known to be hiding at Lexington. As the British departed, Boston Patriots Paul Revere and William Dawes set out on horseback from the city to warn Adams and Hancock and rouse the Minutemen…

Revere arrived in Lexington shortly before Dawes, but together they warned Adams and Hancock and then set out for Concord. Along the way, they were joined by Samuel Prescott, a young Patriot who had been riding home after visiting a lady friend. Early on the morning of April 19, a British patrol captured Revere, and Dawes lost his horse, forcing him to walk back to Lexington on foot. However, Prescott escaped and rode on to Concord to warn the Patriots there. ..

Note:

Riding through present-day Somerville, Medford and Arlington, Revere warned patriots along his route – many of whom set out on horseback to deliver warnings of their own. By the end of the night there were probably as many as 40 riders throughout Middlesex County carrying the news of the army’s advancement. From as far away as Tewksbury and Dracut and perhaps beyond the alarm was heard. Men like Captain John Trull from North Tewksbury who fired a volley of three shots – as agreed – to alert Dracut –  rallied the  local Minutemen and headed to the fight.