This past weekend, I took a jaunt up to Pamplona to see the world-famous Running of the Bulls. First off, just to be very clear, I did not run and was on a balcony about 30 feet above the bulls, so I was perfectly safe the entire time. However, the festival itself was a blast. Most people don't realize that the festival goes on for a week, and the bulls run every morning.
The festival is a combination of the festival of San Fermin, the first bishop of Pamplona, and a local bullfighting festival. Legend has it that they had to get the bulls to the bullring somehow, and the easiest way was to run them through the streets. One year, someone jumped in front of the bulls, and a tradition was born. They were combined, creating a week of festivities, including a live concert and fireworks every night.
Also, everyone in the entire town dresses up in white clothing with a red sash and neckerchief.
You can identify the people who have been at the festival for more than one day because they have red wine staining their clothes, so everyone wears cheap clothes that they throw out afterwards.
The festivities mainly involved drinking truly massive quantities of sangria and the local favorite, kalimotxo. This northern drink is equal parts red wine and coke, which tastes a lot better than it sounds.
So after consuming liters of their favorite alcoholic beverages, everyone stays up all night partying in the streets. All the hotels are full, so people sleep in the open and there are no rules against drinking in public. The result was predictably catastrophic. The bus station was full of hung-over people laying on the ground waiting for their bus home, while the streets were filled with trash, half-drunk drinks, and random people just passed out. (Sorry, no photos of that!)

So, those are the people who stagger to their feet and assemble to watch the running of the bulls. The actual running is surprisingly not as dangerous as you'd think: there have only been 15 deaths since the beginning of record-keeping in 1910. However, it is still crazy to see grown men scrambling to outpace a bunch of bulls as they run to the bull ring.
Basically, the festival of San Fermin is the wildest frat party ever, plus bulls. Quite the combination.