Is missing Mass on Sunday a mortal sin? You have to stop and define mortal sin. First of all, a mortal sin is something which, by its very nature orients us away from God rather than toward God. One of the ten commandments is ‘keep holy the Sabbath Day’. How do you sanctify the Sabbath? By worshipping God. How do you worship God? By going to Mass. If you don’t to this you orient your life away from God and if you continue in this same direction then you will not end up with God. You will end up away from God, and since God is love and light and beauty and truth, then you will end up without love and light and beauty and truth, and that means death. So is missing Mass a mortal sin? Yes.
However, the definition of mortal sin is that it not only has to be grave matter, but you have to know that it is grave matter and there has to be an element of pre meditation to your decision. I’d add, that you also have to continue in your decision without any thought of regret or acceptance of wrongdoing. Also, when deciding a person’s culpability we have to weigh up the circumstances and the intention of the action. Mitigating circumstances and a good intention cannot make a wrong action right, but they can lessen one’s culpability.
If you miss Mass on Sunday because you can’t be bothered, or because something has come up which you believe is more important, then even if it seems a small thing, you have committed a mortal sin. If, however, you miss Sunday Mass because you are traveling and you genuinely can’t get to Mass–even though you tried–it’s not a mortal sin. If you missed Mass because you were sick or because you were looking after a sick old person or child, then it is not a mortal sin.
However, if you miss Mass because you are working–you have decided to put something before God. Even so, if you have to work to support yourself and your family and there really is no way you can get to Mass on a Sunday because of your work commitments, it is possible to ask for a dispensation from your pastor to attend another Mass as soon as possible. But is this ever the case? In most towns there are Mass times from around 5 on a Saturday evening through 5 or even 7 on Sunday night. Does anyone work a shift that keeps them at work that long? I doubt it.
The Catholic Church’s teaching is simple and makes common sense. You have an obligation to go to Mass each Sunday. This helps you to put God first in your life. It helps you to orient your life towards heaven. It helps you to order all things in priority after God. It helps you to walk the path to heaven with confidence and hope. If you miss for a good reason and at no fault of yourself it is not a mortal sin. If you miss through negligence or because you have put something else (anything else) before God, then you’ve oriented yourself away from God and need to go to confession and re-orient your heart to God.
Finally this: why go to confession? Can’t you just say ‘sorry’ to God and be done with it? Several reasons why confession is important: first, the sin of missing Mass intentionally is a sin against the Church as well as against God. Therefore we go to the Church for forgiveness. Secondly, although we make an act of contrition and confess our sins to God on our own, we cannot receive absolution from Holy Mother Church without a priest. Finally, the sin of missing Mass is a formal and ceremonial duty. The restitution for that is to go through the formal and ceremonial sacrament of confession.