Hippolytus, Fragments on Genesis
He did not say "first day." If He had said "first day," He would have been compelled to say "second day" as it came into being. Instead, He had to say not "first" but "one," so that by using "one" He could show this day as cyclical, and the week as completed, yet still a single unit.
Greek
Οὐκ εἶπεν ,,οἡμέρα πρώτη“. εἰ γὰρ πρώτην ἡμέραν εἶπεν, ἀνάγκὴν εἶχεν εἰπεῖν καὶ δευτέραν γινομένην. ἀλλ᾽ ἔδει μὴ πρώτην λέγειν ἀλλὰ ο,,μίων“, ἵνα διὰ τοῦ ,,μίαν““ εἰπεῖν ταύτην δείξῃ κυκλουμένην ὅ καὶ ἑβδομάδα διατελοῦσαν, μίαν δὲ ὑπάρχουσαν.