The Boston Marathon is an annual running event hosted by several cities
in Greater Boston in eastern Massachusetts and managed by the Boston
Athletic Association. It is always held on Patriots’ Day, the third Monday
of April. It is the world’s oldest annual marathon and ranks as one of the 
world’s best-known road racing events and is one of six World Marathon
Majors. Amateur and professional runners from all over the world compete
in the Boston Marathon each year,

Photo of the 8th running of the Boston Marathon.

During the 2013 Boston Marathon, on April 15, two explosions occurred in 
approximately the last 225 yards of the course. Three spectators were killed
and more than 200 people were injured. Entrants who completed at least half
the course and did not finish due to the bombings are to get automatic entry 
in this years race on April 21.