Eleusinia, ancient Greek festival in honour of Demeter (the goddess of agriculture), unconnected with the Eleusinian Mysteries despite the similarity of names. The Eleusinia, which included games and contests, was held every two years, probably in the month of Metageitnion (August–September). Every second festival had a particularly elaborate observance and was known as the Great Eleusinia. Its purpose was thanksgiving and sacrifice to Demeter for the gift of grain.