The Catechism of the Catholic Church states that missing Mass without a serious reason is a “grave sin.”  Thus, someone who skips Mass on purpose, knowing that it is a serious obligation, commits a mortal sin.
One may object: Isn’t this just a matter of canon law?
Not completely. It is true that the stipulation to attend Mass on Sundays and Holy Days is a precept of Church law – in fact the number of Holy Days differs from country to country. However, the Church is not creating a law out of thin air. Rather she is specifying God’s law: to keep holy the Sabbath. We may ask: What is sufficient to keep the Sabbath Holy? Do I need to spend the whole day in prayer or just twenty minutes? The law of the Church exists to clarify this question for Christians, to teach us the “bare minimum” of how to fulfill this command of God. The bare minimum is to attend Mass on Sunday.
A second objection: Would God send someone to Hell just because they don’t have perfect attendance at a Church service?
God doesn’t send anyone to hell – people alienate themselves from God by their own choices. That being said, our Sunday Obligation is not merely a matter of attendance. The obligation is not to “attend” Mass – but to “participate” in Mass. As creatures, we have a serious duty to worship our Creator – without Him, we simply would not exist. The worship most pleasing to God was Christ’s offer of His life to the Father in His sacrifice on the Cross. In the Eucharist, Christ’s sacrifice is made present again – we offer our lives to God together with Jesus. Missing Mass on Sunday is not just missing out on the priest’s homily or receiving Holy Communion. Missing Mass on Sunday is failing to DO something we need to do: worship God.
The Church acknowledges there are “serious reasons” which would excuse someone from participating in Mass on a given Sunday. People have many excuses to miss Mass – which do you think are serious? Which are not?
 Catechism of the Catholic Church 2181.
 In Canada we have two holy days – Christmas (December 25th) and Mary Mother of God (January 1st)
 Canon 1247, Catechism of the Catholic Church
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