I. The reason of Christ suffering at the Paschal Feast
I know indeed, dearly-beloved, that the Easter festival partakes of so sublime a mystery as to surpass not only the slender perceptions of my humility, but even the powers of great intellects. But I must not consider the greatness of the Divine work in such a way as to distrust or to feel ashamed of the service which I owe; for we may not hold our peace upon the mystery of man's salvation, even if it cannot be explained. But, your prayers aiding us, we believe God's Grace will be granted, to sprinkle the barrenness of our heart with the dew of His inspiration: that by the pastor's mouth things may be proclaimed which are useful to the ears of his holy flock.
For when the Lord, the Giver of all good things, says: “open your mouth, and I will fill it,” we dare likewise to reply in the prophet's words: “Lord, You shall open my lips, and my mouth shall show forth Your praise.” Therefore beginning, dearly-beloved, to handle once more the Gospel-story of the Lord's Passion, we understand it was part of the Divine plan that the profane chiefs of the Jews and the unholy priests, who had often sought occasion of venting their rage on Christ, should receive the power of exercising their fury at no other time than the Paschal festival.
For the things which had long been promised under mysterious figures had to be fulfilled in all clearness; for instance, the True Sheep had to supersede the sheep which was its antitype, and the One Sacrifice to bring to an end the multitude of different sacrifices. For all those things which had been divinely ordained through Moses about the sacrifice of the lamb had prophesied of Christ and truly announced the slaying of Christ. In order, therefore, that the shadows should yield to the substance and types cease in the presence of the Reality, the ancient observance is removed by a new Sacrament, victim passes into Victim, blood is wiped away by Blood, and the law-ordained Feast is fulfilled by being changed.
Source: Sermons (New Advent)