The headword occurs at Hesiod, Works and Days 410, in a gnomic utterance which later became proverbial: "not to put off to tomorrow and the day after" (see under epsilon 1294, and cf. epsilon 1295). It is preceded by a hiatus. The old termination in -fi was probably originally adverbial, but is used indiscriminately, as here, in epic language from Homer onward (see Chantraine, Grammaire homérique 234ff.).
The Suda's gloss "after the third [day]" is rather bizarre for "on the third day" (by Greek inclusive reckoning = our "on the second day"). Hesychius epsilon3025 correctly has "to the third [day]", ei)s tri/thn; likewise Athenaeus, Deipnosophists 3.100B [3.56 Kaibel] and a scholion to Aristophanes, Acharnians 172.