Click here to download and listen to this homily (23 minutes) given by Fr Thomas Weise at St Catherine of Siena parish in Petersburg, Alaska. Prepare the way of the Lord, make straight his paths: all flesh shall see the salvation of God. Go to confession Jesus wants your sins as His Christmas gift.
So is missing Mass on Sunday and Holy Days of Obligation a Mortal Sin? The Catechism of the Catholic Church paragraph 1859 states "Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin."
So all three conditions need to be present for the sin to be Mortal: 1) Serious Matter, 2) Knowledge of action as a Serious Matter prior to committing the act, 3) Full consent of the will, freedom to do or not do the act.
If any one of these conditions is defective then there is no mortal sin, yet it involves serious matter and should be confessed and avoided in the future. So is deliberately skipping mass on Sunday/Holiday of Obligation a Mortal Sin? Often Yes, with mitigating circumstances often, No.
Readings: Baruch 5:1-9; Psalm 126; Philippians 1:4-6, 8-11; Luke 3:1-6