There isn't a great deal known about the life of San Gennaro, he is certainly more famous now for the blood miracle that is celebrated three times during the year. On the 16th December it celebrates his patronage of Naples, and the Saturday before the first Sunday in May is for the unification of his relics. The 19th September is in celebration of his feast day.
It is said that if the blood miracle does not happen then the City of Naples will be left without the protection of San Gennaro. For example, in 1980 the blood did not liquefy before the earthquake that took 2,000 lives. One of the earliest recordings of the blood miracle comes from as far back as 1389, with the records officially starting in 1649.
From what we know about the life of San Gennaro, he was from a rich family, and at 15 became a local parish Priest. He became Bishop of Naples at 20 years old, and of his acquaintances was Juliana of Nicomedia and St Sossius. During the persecution of Christians by Emperor Diocletian, he hid his fellow Christians and prevented them from being caught. Unfortunately, while visiting Sossius in jail, he too was arrested.
He was placed in a furnace to be cooked alive, he came out unscathed. He was pushed into the Flavian Amphitheater at Pozzuoli to be eaten by wild bears, who had not eaten in days. Yet the animals refused to eat them, instead licking their toes. Januarius was beheaded along with Sossius and his companions at Solfatara.
The Feast of San Gennaro is a major occasion in Italy, and thousands of Neopolitans will be packing into the Cathedral today to witness the miracle. In defence of the blood miracle of San Gennaro, St. Alphonsus Liguori wrote:
"The Neapolitans honour this saint as the principal patron of their city and nation, and the Lord himself has continued to honour him, by allowing many miracles to be wrought through his intercession, particularly when the frightful eruptions of Mount Vesuvius have threatened the city of Naples with utter destruction. While the relics of San Gennaro were being brought in procession towards this terrific volcano, the torrents of lava and liquid fire which it emitted have ceased, or turned their course from the city. But the most stupendous miracle, and that which is greatly celebrated in the church, is the liquefying and boiling up of this blessed martyr's blood whenever the vials are brought in sight of his head. This miracle is renewed many times in the year, in presence of all who desire to witness it; yet some heretics have endeavored to throw a doubt upon its genuineness, by frivolous and incoherent explanations; but on one can deny the effect to be miraculous, unless he be prepared to question the evidence of his senses.
John Henry Cardinal Newman also attested to the veracity of the miracle of liquefaction:
I think it impossible to withstand the evidence which is brought for the liquefaction of the blood of San Gennaro at Naples.