"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.
If you receive the bread each day, each day is today for you. If Christ is yours today, he rises for you every day. How can this be? "You are my Son, today I have begotten you." Therefore, "today" is when Christ rises.
Source: Catechism of the Catholic Church (Vatican.va)