From Wikipedia:  The festival of San Fermín in the city of Pamplona (Navarre, Spain), is a deeply-rooted celebration held annually from noon 6 July, when the opening of the fiesta is marked by setting off the pyrotechnic txupinazo accompanied by music,^[1] to midnight 14 July, with the singing of the Pobre de Mí. While its most famous event is the encierro, the running of the bulls, the week-long celebration involves many other traditional and folkloric events. It is known locally as Sanfermines and is held in honor of Saint Fermin, the patron saint of Pamplona and co-patron of Navarra. Its events were central to the plot of The Sun Also Rises, by Ernest Hemingway, which brought it to the general attention of English-speaking people. It has become probably the most internationally renowned fiesta in Spain. Origins

The Sanfermines in the medieval period as a private fair and secular fiesta, using for that the dates of religious festivals and using dates of festivals much older such as those of the Basques and Romans. Beginning in the 14th century people concluded certain commercial affairs after the eve of the Feast of Saint John the Baptist, 23-24 June, coinciding with the beginning of summer. Because at these commercial festivals cattle merchants came into town with their animals, eventually bullfighting (corridas) came to be organized as a part of the tradition. Thus was born, sometime probably at the end of the sixteenth century, the genuine first Sanfermines.

Archives document the bull runnings only as far back as the late thirteenth century, but even if one does not know that the bull is a sacred animal in the Mediterranean world, or is unaware of the bull-dancers in Minoan frescoes, an unprejudiced outsider still may detect the remnants of an ancient pre-Christian ritual. At Pamplona, Saint Fermin – who was actually martyred at Amiens – is now sometimes said to have met his end by being dragged through the streets of Pamplona by bulls, a fate also attributed to his mentor, Saint Saturnin of Toulouse. Up to the fifteenth century, the festival was held on Saint Fermin's feast day, 25 September. The Pamplona fiesta was transferred to July in 1592.