What Is A Day?

Ἀβραὰμ ὁ πατὴρ ὑμῶν ἠγαλλιάσατο ἵνα ἴδῃ τὴν ἡμέραν τὴν ἐμήν. Ζητητέον τίνα λέγει ἡμέραν. Φαμὲν τοίνυν, ἐπεὶ φῶς ἀληθινὸν καὶ δικαιοσύνης ἥλιός ἐστὶ τε καὶ λέγεται ὁ Σωτὴρ, ἡμέραν κατασκευάζει ἐπ' ἂν ἀνατείλῃ τινὶ δι' ἀρετῆς καὶ θείας γνώσεως. Ἐπεὶ τοίνυν καὶ τῷ Ἀβραὰμ ὤφθη, καταυγάσας αὐτὸν πίστει εἰς δικαιοσύνην, λελογισμένως μετὰ χαρᾶς ἠγαλλιάσατο, ἰδὼν τὴν νοητὴν τοῦ Κυρίου ἡμέραν. Δίδυμος Αλεξανδρεύς, Εἰς Τὸ Κατὰ Ἰωαννην Εὐαγγελιον Migne PG 39.1652

'Abraham your father rejoiced that he saw my day.' 1   One may ask what day he speaks of. I say that the true light and the sun of righteousness is the Saviour, and this day is made for him who rises up by virtue and Divine knowledge. Thus he appeared to Abraham, enlightening him to righteousness, and with good reason he rejoiced, seeing the rational day of the Lord. Didymus The Blind,  Commentary On the Gospel of John, Fragment 1 Jn 8.56