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Near 2,000 years ago Justin Martyr explained the Paschal mystery in this way.
“The mystery, then, of the lamb which God enjoined to be sacrificed as the passover, was a type of Christ; with whose blood, in proportion to their faith in Him, they anoint their houses, i.e., themselves, who believe on Him. Justin Martyr Dialogue with Trypho Chapter XL
He was saying that Scripture proves two advents of Christ as the Suffering Servant and as the King of kings. That, two advents and why, was the mystery.
As the suffering servant, He would be as the Passover lamb, wholly burned and wholly consumed in the place where God assigns.
and that lamb which was commanded to be wholly roasted was a symbol of the suffering of the cross which Christ would undergo. For the lamb,2055 which is roasted, is roasted and dressed up in the form of the cross. For one spit is transfixed right through from the lower parts up to the head, and one across the back, to which are attached the legs of the lamb. -ibid-
Incidentally, there is no sense that the mystery referred to hidden knowledge. All things were done in the open.
So, to answer the OP, although not a Catholic Church source per se (Justin Martyr), the Pashal mystery refers to Scripture's details about the Passover, how it relates to Christ, and why it includes His two advents. It is about the sacrificial lamb offered once for all time.
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The Paschal Mystery is the greatest mystery of the liturgical year.
Christians don't celebrate the Pasch; that's a Jewish feast.
Fr. Hardon, S.J.'s Catholic Dictionary:
The Jewish feast celebrated annually at God's command to commemorate the deliverance of the Jews from the bondage of Egypt. This deliverance was conditioned on the sacrifice of an unblemished lamb or kid, bones unbroken, whose blood was to be used to sprinkle the doorpost of every Hebrew house on the night before their passage. The deliverance of the Jews from Egypt was a foreshadowing of the Christian Pasch when through the sacrifice of the Lamb of God and the application of the merits of his blood, the human race would be freed from the bondage of the devil and of sin. Good Friday in the early Church was called the Pasch of the Crucifixion, while Easter day was styled the Pasch of the Resurrection, the Sundays from Easter to Whitsunday were always referred to as "after the Pasch." Easter is the Christian Passover.
Dom Guéranger, O.S.B., The Liturgical Year (vol. 7): Paschal Time (book 1), ch. 2 "The Mystery of Paschal Time", p. 14:
Of all the Seasons of the Liturgical Year, Easter-tide is by far the richest in mystery. We might even say that Easter is the summit of the Mystery of the sacred Liturgy. The Christian who is happy enough to enter, with his whole mind and heart, into the knowledge and the love of the Paschal Mystery, has reached the very centre of the supernatural life. Hence it is, that the Church uses every effort in order to effect this: what she has hitherto done, was all intended as a preparation for Easter. The holy longings of Advent, the sweet joys of Christmas, the severe truths of Septuagesima, the contrition and penance of Lent, the heart-rending sight of the Passion—all were given us as preliminaries, as paths, to the sublime and glorious Pasch, which is now ours.
And that we might be convinced of the supreme importance of this Solemnity, God willed that the Christian Easter and Pentecost should be prepared by those of the Jewish Law—a thousand five hundred years of typical* beauty prefigured the reality: and that reality is ours!
*pertaining to a type, which is, according to Fr. Hardon, "A biblical person, thing, action, or event that foreshadows new truths, new actions, or new events."
He continues: "In the Old Testament, Melchizedech and Jonah are types of Jesus Christ. A likeness must exist between the type and the archetype, but the latter is always greater. Both are independent of each other. God's call for the return of the Israelites from Pharaoh's bondage typifies the return of Jesus Christ from his flight into Egypt. In the New Testament the destruction of Jerusalem, foretold by Christ, was the antitype of the end of the world."
(An antitype is "A person or thing typified or prefigured by a biblical person or object."; OED: "That which is shadowed forth or represented by the ‘type’ or symbol.")