For many Catholics, the answer is a no-brainer. But as a revert to the faith not 2 years ago now, I had no clue. And so I thought I'd do my part to help out:
One of the things that makes the Roman Catholic Church distinct from other Christian denominations is that we have days throughout the year– in addition to Sundays– when we’re obligated to attend Mass.
The six Holy Days are: Solemnity of the Immaculate Conception (December 8th), The Nativity of the Lord (Christmas), Solemnity of Mary the Mother of God (January 1st), Ascension of the Lord (the date moves with Easter), Solemnity of the Assumption (August 15th), and the Solemnity of All Saints (November 1st). Notice that Ash Wednesday isn’t on the list, even though it’s one of the busiest days of the year for most parishes.
Obligated? You mean... you have to go to Mass?
Well... bluntly... yes if you're one who takes the faith seriously:
2041 The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the very necessary minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbor:
2042 The first precept ("You shall attend Mass on Sundays and holy days of obligation and rest from servile labor") requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days.82
If that's too hard to understand, here's an explanation for dummies:
Holy days are like Sundays in that Catholics must attend Mass, and if possible, refrain from unnecessary servile work. Some Catholic countries, such as Italy, Spain, and Ireland, give legal holiday status to some of these holy days, so people can attend Mass and be with family instead of at work.
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 can not 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.
That whole piece is well worth reading.
The bottom line is pretty simple. If you're Catholic and deem yourself faithful, get thy buttocks to Mass tonight.