The article argues that ἐνιαυτός "anniversary" is a back-formation on the adjective ἐνιαύσιος, attested in Mycenaean as a proper name. It must be compared with the adverb πέρυσι "last year". The sequence -υτι-ος (-υσι-ος in assibilating dialects) preserves the locative of the zero grade *ut- of ϝέτος "year". The αυ- is assumed to be the adverb αὖ "anew, back", the anniversary being the "returning year". The compound ἐνιαύσιος relies on a phrase *ἐνι αὐτι,, with ἐνι being the locative of an old word for "year" found in the compounds δίενος, τρίενος (Theophrastus). The phrase was univerbated when a derived adjective ἐνιαύτι-ος was formed. Afterwards, after couples like πλοῦτος / πλούσιος, from ἐνιαύσιος was derived the noun ἐνιαυτός as a back-formation.