Lewis & Short

Ĕleuthĕrĭus, i, m., = Ἐλευθέριος (Liberator), the surname of a god: Liber, Arn. 6, p. 207.
Hence, Ĕleuthĕrĭa, ōrum, n., the festival of Jupiter Eleutherius, feast of liberty, celebrated in honor of the victory at Plataea, Plaut. Pers. 1, 1, 29.