Today is the feast of the bishop and martyr, St. Januarius (+305AD), or San Gennaro, patron of the city of Naples in southern Italy.
St. Januarius by Caravaggio (+1610AD). The head in the corner reflects the manner in which the saint met his martyrdom.
St. Januarius, Bishop of Beneventum, was martyred in 305AD during the persecution of Diocletian, and this by beheading. While we don't know a tremendous amount about his life, he is certainly famous for his patronage of the city of Naples, and of the liquefaction of a phial of his blood kept at the Cathedral on his feast day, today.
For a bit more about the saint, here is the patron saints index entry: Catholic Saints Info: St. Januarius of Naples
This is the text of the Acts of the Hieromartyr Januarius, which purports to elaborate on the details of his life and martyrdom: Catholic Saints Info: Life of Januarius
Finally, the Old Catholic Encyclopedia goes into a more detailed discussion of the annual miracle of the blood: Old Catholic Encyclopedia: St. Januarius
The Cardinal-Archbishop of Naples holds the phial containing the blood. It seems the waving of the white handkerchief is the signal that it has liquefied -- which usually, but not always, happens.
Here is an article, with some great photos, of the liquefaction of the blood of St. Januarius: Catholic Herald (UK): "The day I saw a saint’s blood become liquid" (2011)
Here is an account from an Italian source on the liquefaction as it happened in 2013: Gazzetta del Sud Article (2013)
Here is a link to a blog with some great video and information on the miracle, including from 2014 Fr. Z's Blog: The blood of San Gennaro liquefied!
Early, in March 2015, there was a unique occurrence of the miracle in the presence of Pope Francis. It had never taken place at on "off time" before: Pope Francis in Naples & San Gennaro Miracle
UPDATED: Here is an article on the blood this year, 2016: National Catholic Register: Blood of St. Januarius
UPDATED II: Yet another article on this year's event, 2016: Crux: Famed Blood Miracle