Feast Day is September 19
Tradition has it that St Januarius was bishop of Benevento (a town near Naples) later to die a Christian martyr beheaded at the hands of the Emperor Diocletianus, in 305 AD at Pozzuoli.
Ceremonies in his honour were instituted by archbishop Orsini of Naples in 1337 No mention of the liquefying blood was made however, until 1389, when on August 17, the phenomenon was first reported. A chronicle of Naples written in 1382 describes the Januarian cult but still makes no mention of either the miracle or the relic. It is very likely that "the blood of Januarius" is one of the many relics to materialize during the Middle Ages
Today the rite is on one hand still encouraged (is performed by the archbishop in the Naples cathedral); on the other hand it was never officially declared a miracle by the Catholic Church, which leaves scientists free to express their opinions.
A sealed glass vial containing a dark unknown substance, allegedly the clotted blood of San Gennaro (St Januarius), is shown several times a year to a packed crowd in the Cathedral of Napoli (Naples). Whilst the container is being handled during a solemn ceremony, the solid mass suddenly liquefies before everybody's eyes.
This well-documented phenomenon is still regarded as unexplained by believers and sceptics alike. Noted parapsychologist Hans Bender defined it the paranormal phenomenon with the best and historical documentation; physicist Enrico Fermi seems to have expressed interest as well.
It is also one of the few recurrent non-medical, physical "miracles" that might be studied scientifically.
Parts taken from Prof. Steno Ferluga and Ms Gillian Jarvis: http://www.cicap.org/new/articolo.php?id=101014.