Summary: Three times a year – on his feast of St Januarius, on the day of the translation of his relics and on the day he saved Naples from an eruption on Mount Vesuvius – the liquefaction of his blood of is said to occur.
Patrick Duffy gives the background.
Fourth century martyr at Naples
Januarius was a 4th century bishop of Beneventum, who with his companions suffered martyrdom in the persecution of Diocletian at Naples. It is said he was first thrown into a fiery furnace and remained unscathed. Then thrown to the lions in the arena, they refused to touch him. Finally he was beheaded. The Christian women collected his blood in a glass vial and placed it in his tomb.
Liquefaction of his blood
The first recorded liquefaction of his blood was recorded in the year 1389 and since then countless pilgrims have witnessed this unexplained occurrence. The phenomenon has been the subject of intense dispute and scientific examination for centuries. The Church has never made any official declaration that it is a supernatural event. But even sceptics admit that something happens.
What is involved is a dark solid mass held in a glass vial kept in the treasury chapel of Naples cathedral. Three times a year a priest brings it out with a reliquary said to contain the saint’s skull, holds it up and turns it as the people pray. After a period of anything from two minutes to an hour, it appears to become red and to bubble – or not. If it does, the priest proclaims the miracle and all sing the Te Deum in thanksgiving. If not, this is taken as a kind of prophetic warning.
Invoked against volcanic eruptions
In 1631 an eruption on nearby Mount Vesuvius threatened. The people prayed to St Januarius to spare them. The flow of lava abated and the city was saved. Ever since, St Januarius has been invoked against volcanic eruptions. He is also the patron saint of blood banks.