The Chapel of the Treasure in Naples Cathedral preserves what is believed to be the blood of Saint Januarius, according to tradition a bishop martyred in the fourth century. Currently the blood liquefies in particular periods of the year, in conjunction with liturgical feasts connected to the saint. This chapter intends to reconstruct the genesis of the liquefaction ritual and to demonstrate how the “periodicity” of the miracle is the outcome of previous practices of the exhibition of the relic.