At Athens, during the celebration of the Eleusinian mysteries, the main public event was the procession from Athens to Eleusis along the Sacred Way, in which the crowd shouted Iakch' o Iakche, it was presumed in invocation of a deity Iacchus who was being celebrated together with Demeter and Persephonē. He was variously said to be the son of Demeter or of Persephone, or to be Dionysus by another name, because of the similarity between his name and Bacchus (i.e. Dionysus). Herodotus recounts that after the Persians had conquered the Greek mainland shortly before the battle of Salamis a great cloud of dust ‘as if caused by thirty thousand men’ was seen coming from the direction of Eleusis, and the Iacchus shout was heard proceeding from it. The cloud moved towards Salamis, where the Greek army was stationed; the festival prevented by war was being celebrated supernaturally, with good omen for the Athenians. In Italy Iacchus was occasionally identified with Liber, as in the temple of Cerēs on the Aventine.

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