I could not possibly do a better job than the Catholic Tradition website (from whence I borrowed the beautiful painting, above) in re-telling the story of today's saint ~ and his is a story of such fortitude, piety and wondrous miracles, that I highly suggest running over to read the whole tale.
But, in the interest of piquing your curiosity, here are some highlights: St. Januarius was the bishop of Beneventum, Italy, during the reign of Diocletian. He was tortured for the Faith, then sent into the arena with the wild beasts to be devoured ~ but the beasts knelt before the holy bishop, sparing his life as the evil Romans would not do. After other amazing miracles, our saint was finally beheaded and his soul flew to its eternal reward in heaven.
His body was interred first at Beneventum, then, in the monastary of Monte Vergine, and finally at the principal church at Naples (the Cathedral of San Genarro). The holy relic has always been accompanied by miracles, one most notable being the quieting of Mount Vesuvius, thus sparing its prayerful neighbors from iminent destruction.
But, the most famous and enduring of the wonders of St. Januarius is the miracle of the liquification of his blood every year on the anniversary of his martyrdom. When a vial of the saint's blood is brought into the vicinity of the relic of his severed head, the contents, which, during the rest of the year, are dried and solid in the crystal reliquary, become liquid. God still honors and utilizes this holy saint to turn and edify souls by the proof of His power. This amazing miracle which has occured throught the centuries can still be seen today!
Praise be to God in his Angels and in His Saints!
Note: Removed the pic here, as it was pointed out to me that it wasn't of the actual miracle. Oops!