It is always held on Patriots' Day, the third Monday of April. Begun in 1897, the event was inspired by the success of the first marathon competition in the 1896 Summer Olympics.
The Boston Marathon is the world's oldest annual marathon and ranks as one of the world's best-known road racing events. It is one of six World Marathon Majors. Its course runs from Hopkinton in southern Middlesex County to Copley Square in Boston.
The Boston Athletic Association (B.A.A.) has organized this event since 1897, and it has been managed by DMSE Sports, Inc. since 1988. Amateur and professional runners from all over the world compete in the Boston Marathon each year, braving the hilly Massachusetts terrain and varying weather to take part in the race.
The event attracts 500,000 spectators each year, making it New England's most widely viewed sporting event. Though starting with 15 participants in 1897, the event now attracts an average of about 30,000 registered participants each year, with 30,251 people entering in 2015.The Centennial Boston Marathon in 1996 established a record as the world's largest marathon with 38,708 entrants, 36,748 starters, and 35,868 finishers.
According to wikipedia