Read this: One of the best known sporting events in the city is the Boston Marathon, the 26.2-mile (42.2 km) race which is the world's oldest annual marathon, run on Patriots' Day in April. On April 15, 2013, two explosions killed three people and injured hundreds at the marathon. Another major annual event is the Head of the Charles Regatta, held in October.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): When is the head of Charles Regatta held?
October