Rather than just approaching this question from the angle of "missing Mass is a sin," we should first call to mind the importance of the Mass. Each Sunday, we gather together as a Church with hearts filled with joy to worship Almighty God. We remember and profess our faith once again in the mystery of our salvation,that Jesus Christ, the Son of God, suffered, died, and rose for our salvation. The saving actions of Holy Thursday, Good Friday, and Easter Sunday coalesce in the Holy Sacrifice of the Mass. The Constitution on the Sacred Liturgy of the Vatican Council II asserted, "For it is the liturgy through which, especially in the divine sacrifice of the Eucharist, 'the work of our redemption is accomplished,' and it is through the liturgy, especially that the faithful are enabled to express in their lives and manifest to others the mystery of Christ and the real nature of the true church" (#2).
Moreover, at Mass, each faithful Catholic is fed with abundant graces: First, we are nourished by the Word of God — God's eternal truth that has been revealed to us and recorded under the inspiration of the Holy Spirit. We then respond by professing our Holy Catholic Faith as presented in the Creed, saying not simply “I believe" as a singular person, but “we believe" as part of the Church.
Second, if we are in a state of grace, then we have the opportunity to receive our Lord in the Holy Eucharist. We firmly believe that our Lord is truly present in the Holy Eucharist, and that we receive His body, blood, soul, and divinity in Holy Communion. Not only does the Holy Eucharist unite us intimately with the Lord, but also unites us in communion with our brothers and sisters throughout the universal Church. The Holy Eucharist is such a precious gift!
With this in mind, no one should simply think of attending Mass as fulfilling an obligation. To attend Mass is a privilege, and any faithful Catholic should want to attend Mass. Our perspective should not be, "I have got to do this"; rather, we should think, "I get to do this."
Nevertheless, because the Mass offers such precious gifts, provides the nourishment of great graces, and unites us as a Church, we do indeed have a sacred obligation to attend Mass. Remember that the Third Commandment stated, "Keep Holy the Sabbath." For the Jewish people, the Old Testament Sabbath was on Saturday, marking the "Day of rest" after creation. For Christians, we have always kept holy Sunday, the day of the resurrection. Just as creation unfolded on the first day of the week with God commanding, "Let there be light," our Lord, the Light who came to shatter the darkness of sin and death, rose from the dead on that first day marking the new creation.
Given how precious the Mass is plus the Old Testament precedent which was rightly adapted by the Church, the Code of Canon Law (#1246) proscribes, "Sunday is the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost holy day of obligation in the universal Church." Moreover, "On Sundays and other holy days of obligation, the faithful are bound to participate in the Mass..." (#1247). Therefore, the Catechism teaches, "Those who deliberately fail in this obligation commit grave sin" (#2181), and grave sin is indeed mortal sin. Recently, our Holy Father, Pope John Paul II, repeated this precept in his apostolic letter Dies Domini (Observing and Celebrating the Day of the Lord, #47, 1998).
Of course, serious circumstances arise which excuse a person from attending Mass, such as if a person is sick, has to deal with an emergency, or cannot find a Mass to attend without real burden. A pastor may also dispense a person from the obligation of attending Mass for serious reason. For instance, no one, including our Lord, expects a person to attend Mass who is so sick he cannot physically attend Mass; there is no virtue in further hurting one's own health plus infecting everyone else in the Church. Or, in the case of a blizzard, a person must prudently judge whether he can safely travel to attend Mass without seriously risking his own life and the lives of the others. When such serious circumstances arise which prevent a person from attending Mass, he should definitely take time to pray, read the prayers and readings of the Mass in the Missal, or watch the Mass on television and at least participate in spirit. Keep in mind when such serious circumstances arise, a person does not commit mortal sin for missing Mass.
In examining this question, a person must really reflect on how valuable the Mass and the Holy Eucharist is. Every day, faithful Catholics in the People's Republic of China risk educational and economic opportunities and even their very lives to attend Mass. In mission territories, people travel many miles to attend Mass. They take the risk and they make the sacrifice because they truly believe in the Mass and our Lord's presence in the Holy Eucharist.
When a person negligently "bags Mass," to go shopping, catch-up on work, sleep a few extra hours, attend a social event, or not interrupt vacation, the person is allowing something to take the place of God. Something becomes more valuable than the Holy Eucharist. Sadly, I have known families who could walk to the Church but choose not to attend Mass; ironically though, they send their children to the Catholic school. Yes, such behavior really is indicative of turning one's back on the Lord and committing a mortal sin.
God must come first in our lives. On Sunday, our primary duty is to worship God at Mass as a Church and to be nourished with His grace. The Didascalia, a third century writing, exhorted, "Leave everything on the Lord's Day and run diligently to your assembly, because it is your praise of God. Otherwise, what excuse will they make to God, those who do not come together on the Lord's Day to hear the word of life and feed on the divine nourishment which lasts forever?" Yes indeed, what excuse will they make?