St. Patrick's Day is usually remembered as a story of banishment. A saint arrives, the serpents vanish, and Ireland is cleansed of something older and darker.   Yet only a few days earlier in the ancient Mediterranean calendar stood a very different observance.   The Diasia honored Zeus Meilichios, an underworld form of Zeus associated not with thunder and kingship, but with the deep earth and the spirits that dwell beneath it. In many depictions he appears not as a bearded s