(Taken from Notes Made at the Conferences of Dom Prosper Guéranger).
Et resurrexit tertia die secundum Scripturas. On the third day, He rose again, as the prophecies had foretold, specially that of the Prophet Jonas. Our Lord Himself had said: this wicked and perverse Generation seeketh a sign, but one shall not be given to it, unless it be that of Jonas the Prophet, nisi signum Jonae prophetae (S. Matth. xii. 39, S. Luc. xi. 29). For just as Jonas was in the whale’s belly, three days and three nights, so shall the Son of Man be three days and three nights in the heart of the earth.
Et ascendit in coelum. He ascended into Heaven. The Word of God, coming down on earth to be made man, quitted not His Father’s Bosom. In this place, it is said that He ascended into Heaven, meaning that His humanity actually went up thither, and that there It was enthroned for eternity.
Sedet ad dexteram Patris. He is seated at the Right Hand of the Father, as Master and Lord. Indeed, He was always there according to His Divine Nature, but it behoved Him to be there also according to His Human Nature, and this is what is expressed by these words. In fact, this was a necessity, because the Human Nature being united to the Divine Nature in one and the same Person, which is the Person of God the Son, it can in all truth be said: the Lord is seated at the Right Hand of the Father. David foretold this, saying: Dixit Dominus Domino meo: Sede a dextris meis. [The Lord said to my Lord, sit at my right hand.] (Ps. cix. 1). This is a proof of the intimate union which exists of the Divine Nature with the Human, in the Person of Our Lord. For this reason, the Psalm cix. is essentially the Psalm of the Ascension, because that was truly the moment when the Lord, the Father, said to the Lord, the Son: Sit Thou at My Right Hand: Sede a dextris meis.