Ad Orientem

"Praying toward the East is handed down by the holy apostles, as is everything else. This is because the comprehensible sun of righteousness, Christ our God, appeared on earth in those regions of the East where the perceptible sun rises, as the prophet says: 'Orient is his name' (Zech. 6:12); and 'Bow before the Lord, all the earth, who ascended to the heaven of heavens in the East' (Cf. Ps. 67:34); and 'Let us prostrate ourselves in the place where His feet stood' (Cf. Ps. 67:34); and again, 'The feet of the Lord shall stand upon the Mount of Olives in the East' (Zech. 14:4). The prophets also speak thus because of our fervent hope of receiving again the paradise in Eden, as well as the dawn of the brightness of the second coming of Christ our God, from the East."

St. Germanus of Constantinople.