The festival of San Fermín, held every July in Pamplona, Spain, since the 16th century, has its origins in religious tradition, honoring a Catholic saint now nearly 2,000 years martyred. But the celebrations have become increasingly secular over the years, with round-the-clock revelry—and nostalgia for a world Ernest Hemingway so vividly painted in The Sun Also Rises—drawing visitors from around the world. The festival’s most enduring legacy is the running of the bulls, or encierro, from the Spanish word for to corral or enclose.