Nail fixed in the Temple
(of Jupiter). On September 13th a nail was annually driven into the wall of the temple of Jupiter. This was originally done to tally the year, but subsequently it lapsed into a religious ceremony for warding off calamities from the city. Originally the nail was driven in the wall by the praetor maximus, subsequently by one of the consuls, and lastly by the dictator. (See Livy, vii. 3.).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894