Ernest Hemingway was fascinated by the running of the bulls and bullfights. His depictions of Spain, festivals and the bulls likely increased the popularity of the events.
Thought to originate in the 13th century, La Fiesta de San Fermín originated in 1591 when three festivals – the San Fermín festival, town fair of Pamplona and a celebration of bullfighting were combined into one.
Source: AS USA, July 7, 2023. Link.
Also see: Festival of San Fermin – La Fiesta de San Fermín, 200 words-a-day.com. Link. The author shares history of the festival, why runners dress as they do and more.