Little is known about St. Januarius other than that he is an Italian saint who died under the persecution of Diocletian. When being thrown to the savage beasts, it is said that the animals did not attack. Instead St. Januarius and companions had to be beheaded.  A container of his blood was taken back to Naples, France in the early 4th century. Since that time, the blood has periodically liquified from its coagulated state inside its glass container. 

The three days in which it can be liquified are: the first Saturday before the first Sunday in May (celebrate the original delivery of the blood to Naples), St. Januarius’ feast day (September 19th), and the day Naples was spared from a volcano in the 17th century due to his intercession (December 16th). The blood does not always liquify, but does on most days. It also can liquify on other days for various reasons (like the presence of the Pope). The last time it did not liquify was in December 2016.