Q. Is it a mortal sin to miss Mass on Sundays and Holy Days?
The 3rd Commandment tells us to keep the Lord's Day holy. In Old Testament times, God's people stopped working and gathered together for worship on the Sabbath, which was Saturday. However, the early Christians rested and came together to rejoice on Sunday because Jesus, Our Lord and Savior, rose from the dead on Easter Sunday morning. Sunday is not only a day of worship but a day of joy and family closeness.
Now as to whether it is a mortal sin or not. The three criteria for determining if a sin is mortal were described under the question on mortal and venial sin. It is a grave matter to miss Mass on Sunday and Holy Days. If we have full knowledge that it is a sinful matter to violate one of the Ten Commandments because it violates God's law, the second condition is met. If we realize that the 3rd Commandment requires us to gather together to worship and we know that it is a grave sinful matter to violate the Commandments, but the car won't start and there is no other way to get to Mass, the third condition has not been met. If however, we simply miss Mass because there is something else we would rather be doing (like playing golf, going to the beach, etc.) then a deliberate act has taken place and a mortal sin has been committed. It's like saying "I don't love God enough to spend some of my time with Him."
Recommended reading:
● Catechism of the Catholic Church, paragraphs 1857-1861
● "Remember The Sabbath . . . Keep It Holy", Booklet #36, Catholic Information Service, Knights of Columbus, P.O. Box 1971, New Haven CT 06521, 1967