415 BC, just prior to the launching of the Sicilian expedition, the largest and best-equipped fleet yet assembled by the Athenians, intending to sail to Sicily and conquer some if not all of it. All over Athens (at people’s front doors, at crossroads, at one corner of the Agora for example) were Hermai, life-size or larger stone pillars with Hermes’ head carved at the top and genitals at the front. Hermes, the messenger god, oversaw comings and goings. On the night before the expedition was due to sail, someones mutilated a large number of these Herms, defacing and castrating them.
See Thuc. 6.27. D M MacDowell Andocides on the Mysteries.