The patron of San Gennaro Vesuviano is Saint Januarius, after whom the municipality is named; his feast falls on 19 September.
Marquis Scipione Pignatelli founded the town's fair in the 17th century, the oldest in southern Italy, leaving his estates to the Franciscan friars on condition that they renew it every year; on 19 September trading would stop for the Mass held in the saint's memory, celebrated by the father superior from the balcony of the friary.