"At the Last Supper, on the night he was betrayed, our Savior instituted the Eucharistic sacrifice of his Body and Blood. This he did in order to perpetuate the sacrifice of the cross throughout the ages until he should come again, and so to entrust to his beloved Spouse, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a Paschal banquet 'in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us.'" Catechism of the Catholic Church 132
Why must we attend Mass on Sundays and Holy Days?
As Catholics, we know that on Sundays and other holy days of obligation we, the faithful, are bound to participate in the Mass—but why? To answer, let’s take a look at what our mother Church has to say…
The Sunday Eucharist [what takes place at Mass] is the foundation and confirmation of all Christian practice. For this reason, the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.
Participation in the communal celebration of the Sunday Eucharist is a testimony of belonging and of being faithful to Christ and to his Church. The faithful give witness by this to their communion in faith and charity. Together they testify to God's holiness and their hope of salvation. They strengthen one another under the guidance of the Holy Spirit.
Just as God "rested on the seventh day from all his work which he had done," human life has a rhythm of work and rest. The institution of the Lord's Day helps everyone enjoy adequate rest and leisure to cultivate their familial, cultural, social, and religious lives.
These paragraphs were taken directly from the Catechism of the Catholic Church paragraphs 1280-1284. If you’re ever interested in what the Church teaches on a particular aspect of our faith or morality, refer to the Catechism first! You will be surprised by the clarity and beauty of some of the passages and explanations.