This festival celebrates Saint Ansanus, martyred under the co-emperors Diocletian and Massiminian. A child of the noble Roman family of the Anici, Ansanus was denounced by his father for having received baptism. He was arrested and tortured, but managed to escape and reach Siena, of which he became apostle and evangeliser: before long he had converted much of the city to Christ by way of his striking prophecies and testimonies. The governer had him arrested, tortured and decapitated on 1 December 303. In Siena the Festa di Sant'Ansano marks the beginning of the year for the contrade, all seventeen of which make their way in a solemn procession from Piazza del Campo to the Duomo, where, at the altar of saint, the Archbishop of Siena presides over a celebration of Holy Mass.

Source: comune.siena.it