The deeply religious festival of San Fermin, in honor of a 3rd-century Christian convert, and the famed Encierro, the running of the bulls, have been held in Pamplona, Spain, since the 1400s. The latter began as a non-religious fair centered on the arrival of cattle merchants in summer. Hurrying their livestock through the streets to market eventually evolved into a competition -- now, what we know as running of the bulls.

Source: Travel Channel