Every July 2nd and August 16th, Siena switches from a relaxing escape to a neighborhood battle for the Palio horse race. Il Palio is a bareback horserace held twice a year to honor the Virgin Mary. There are 17 neighborhoods in Siena, but the catch is there are only 10 spots in each race (there is not enough space for all 17 to run each time). Let's see, 10 spots, two races, 17 neighborhoods - something doesn't come out even. Every year, the captains of the race decide who the lucky 3 neighborhoods are that will get to run in both races. Needless to say, it is a source of pride to run both races, and there is cutthroat competition between the neighborhoods to be the ones chosen.
But I'm getting off on a tangent, so back to the experience. In 1998, we were lucky enough to be in Siena at the time of the Palio. The Campo is converted to a racetrack, and tall bleachers line the entire edge of the track. (The shopkeepers are tolerant of the bleachers which block entrance to their shops). The air is alive with the drums and chants of the different neighborhoods. If you head in the direction of the drums, you will stumble upon truly local experience as the neighborhood residents cheer their horse & rider. Follow the crowd for a little while. It will be a memorable experience.
The Palio itself is rather a crazy blur, unless you reserve a bleacher seat (about $150 a piece, reserve at least 6 months in advance) you'll be crowded into the Campo with most of the local residents. Forget trying to see much of the race unless you can push yourself to the edge of the track. If you can see, pick a horse & cheer it on, if you can't, just cheer when the people around you cheer.
The Palio is an experience unlike any other. If you can wrap it into your schedule, your trip will be all the richer.