Every year on May 1 and again on September 19, the holy blood of St. Gennaro liquifies. Located in a reliquary in Naples, Italy, St. Gennaro's blood has been changing from its solid state on cue for hundreds of years. Sure, there are 16 other vials of blood that do the same located in Naples alone, but St. Gennaro's is important: Naples residents look at it as a sign of the coming year. In times when the blood hasn't liquified, plague, earthquakes, and an eruption by nearby Vesuvius have followed.
The author details the violent history of St. Gennaro, as well as a description of this year's ceremony complete with the big event:
With a magician's flourish, the cardinal turned the reliquary upside down and held it up to the light, squinting into the bottle. There was a collective intake of breath. From where I was standing, the blood could clearly be seen to have liquefied and was trickling down the inside of the bottle. The cardinal turned his eyes heavenward, as if in thanks. "It has started," he said.
-Telegraph.co.uk: City where miracles are in the blood
(link ourtesy The Anomalist)
