The liquefaction of the blood of St. Januarius (San Gennaro) in the Cathedral of Naples, Italy is an extraordinary miracle of the Church that has just occurred again, on September 19, 2016, the saint's feast day. This miracle has been occurring up to 18 times each year for the past 600 years. It is only one of a number of blood miracles that have taken place with blood that was collected soon after the death of certain martyrs.

St. Januarius was a bishop and martyr who lived in Italy in the third and fourth centuries during the reign of the Roman Emperor Diocletian. It was a common practice of early Christians to collect the blood that was shed by the martyrs as a relic. The persecution of Christians at that time led to the imprisonment and executions of many, and St. Januarius journeyed to visit a deacon who had been imprisoned for his faith. Januarius and his companions were imprisoned and thrown to the wild animals. When the animals failed to attack the men, they were beheaded.

The blood in the reliquary usually liquefies now on three specific days: September 19 (the saint's feast day), December 16 (the anniversary of the eruption of Mt. Vesuvius in 1631), and the Saturday prior to the first Sunday of May (commemorating the transferring of the relics to Naples). In March 2015, the blood liquefied when Pope Francis visited Naples — the first time the blood had liquefied in the presence of a pope in over 150 years.

When St. Januarius' remains were first transferred to the catacombs in Naples, the dried blood liquefied for the first time. For at least the last 600 years the blood has been liquefying several times a year. His body and a vial of his blood are entombed in the city’s cathedral. Since the 14th century a procession through the cathedral has taken place whenever the miracle takes place, with a priest holding the flasks of liquid blood.

On the feast days of the saint the silver and crystal reliquary is held overhead by its handle by a priest or cardinal in full view of all in the cathedral. Prayers are recited and the priest makes sure that the glass is not touched. After an interval of a few minutes, the mass of coagulated blood is seen to gradually detach itself from the walls of the glass reliquary and to liquefy, frequently bubbling and frothing. The reliquary is then circulated among the faithful for veneration. The priest announces, “The miracle has happened,” and the people begin to sing the ‘Te Deum’.

Every imaginable test and every skeptic's theory has been elaborately studied over the years, with the result remaining that the blood of St. Januarius changing forms is scientifically inexplicable.

Neopolitans say that, on the rare occasions whenever the blood has failed to liquefy on the usual days, trouble has followed — political persecution, war, famine, or disease. Because of this the people of Naples and the world look to the blood of St. Januarius each year and breathe a sigh of relief when the vial of his dried blood turns to liquid — as it did in 2016, as proclaimed by Cardinal Crescenzio Sepe of Naples.