Encierros in Aldeanueva

The Running of the Bulls (in Spanish encierro, from the verb encerrar, to fence in, to lock/shut up, to pen)[1] is a practice that involves running in front of a small group of bulls (typically a dozen) that have been let loose on a course of a sectioned-off subset of a town's streets. The most famous running of the bulls is that of the seven-day festival of Sanfermines in honour of Saint Fermin in Pamplona,[2] although they are held in towns and villages across Spain, Portugal, in some cities in Mexico,[3] in San Jose Festival held in Trujillo, Peru, Mesquite, Nevada,[4] southern France during the summer.

The origin of this event comes from the need to transport the bulls from the off-site corrals where they had spent the night, to the bullring where they would be killed in the evening. Youngsters would jump among them to show off their bravado. In Pamplona and other places, the six bulls in the event are still those that will feature in the afternoon bullfight of the same day.

Spanish tradition says the true origin of the run began in northeastern Spain during the early 14th century. While transporting cattle in order to sell them at the market, men would try to speed the process by hurrying their cattle using tactics of fear and excitement. After years of this practice, the transportation and hurrying began to turn into a competition, as young adults would attempt to race in front of the bulls and make it safely to their pens without being overtaken. When the popularity of this practice increased and was noticed more and more by the expanding population of Spanish cities, a tradition was created and stands to this day.[5]

From: http://en.wikipedia.org/wiki/Encierro