Elite athletes from around the world compete in the Head of the Charles Regatta, the biggest 2-day rowing (sometimes called "sculling" or just "crew") race in the world.
Held on the 3rd weekend in October when fall foliage colors begin to reach their peak along the Charles River where the races take place, the Head of the Charles Regatta attracts over 8,000 local and world-wide rowers.
Here in Boston, a massive crowd of up to 300,000 spectators turn out to watch along both sides of the river plus nearby bridges. My family usually heads to the Boston University Bridge, as the crowds are somewhat thinner there.
You can find a course map near the bottom of this page.

Women's crew team competing in Head of the Charles Regatta

Three members of the Cambridge Boat Club started the Head of the Charles Regatta back in 1965, and the race has grown ever since. Members of racing clubs, college crew teams, and even high school teams converge to compete to be the best in their class.
A "head," in England, is a type of regatta, or boat race, in which boats depart at 15-second intervals on a 3-mile race. The winner of each race is called the "head" - or, in these races, the "Head of the Charles."
Race events start at roughly 15 second intervals - so it's an action-packed day with record-breaking numbers of sculls filling the river. At just about any time, you can see numerous boats on the water.
In addition to the races, the event features displays by boat builders, a rowing and fitness expo, other sponsors, and, of course, food. Reunion Village, an area filled with tents set up by participants and sponsors, provides a lively social and networking scene.
In addition, many of the university and private boat houses along the river host open houses, which can be a lot of fun to visit, especially since many of the sponsors offer free giveaways.

Keep in mind that October weather in Boston can be tricky. During recent years, regattas have experienced everything from spectacular sunshine and blue skies to drenching downpours to heavy fog. We even had a freak snowstorm a few years ago.
When the weather is good, bring your camera because you'll see exciting action to photograph on the river, plus plenty of gorgeous fall foliage. When the weather is bad . . . well, personally I find alternate indoor activities unless someone I know is racing, but committed fans will bring their umbrellas and windbreakers, and enjoy the races just the same.

Spectators watching Head of the Charles Regatta from the BU Bridge