Catechism of the Catholic Church §2836 — excerpt
"This day" is also an expression of trust taught us by the Lord, which we would never have presumed to invent. Since it refers above all to his Word and to the Body of his Son, this "today" is not only that of our mortal time, but also the "today" of God.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Ambrose, De Sacr. 5, 4, 26: PL 16, 453A