In a nutshell: all of the above.
The holy Mass is a sacrifice and a celebration. We commemorate the Passion and resurrection of Christ.
The holy Mass is certainly a great mystery. Many volumes could be written about it and would never exhaust its richness. The Church and the Saints have written extensively about it over the centuries. I will not attempt to survey the entire literature of what has been said about the Mass. I will limit myself to a very short quote from paragraph 1330 of the Catechism, which provides the basic answer to the question posed in the title of this post.
Paragraph 1330 is situated in a section of the Catechism that enumerates the various names given to this wonderful sacrament. In paragraph 1330, we read the following (italics are in the original text, additional emphasis added by me):
"[This Sacrament is called] the memorial of the Lord's Passion and Resurrection.
The Holy Sacrifice, because it makes present the one sacrifice of Christ the Savior and includes the Church's offering. The terms holy sacrifice of the Mass, "sacrifice of praise," spiritual sacrifice, pure and holy sacrifice are also used, since it completes and surpasses all the sacrifices of the Old Covenant.
The Holy and Divine Liturgy, because the Church's whole liturgy finds its center and most intense expression in the celebration of this sacrament; in the same sense we also call its celebration the Sacred Mysteries. We speak of the Most Blessed Sacrament because it is the Sacrament of sacraments. The Eucharistic species reserved in the tabernacle are designated by this same name."
As you can see, the Church has answered our question quite succinctly in paragraph 1330.
I encourage you to read the entire section on the mass as explained in paragraphs 1322 to 1419 of the Catechism.
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