It is both a precept of the Church and Church law that Catholics must worship God on Sunday and on Holy Days of Obligation by participating in the Holy Mass. This follows from the fact that in the Mass it is Christ Himself who worships the Father, joining our worship to His. In no other way is it possible to adequately give thanks (Eucharistia) to God for the blessings of creation, redemption and our sanctification than by uniting our
offerings to that of Jesus Christ Himself. Following the example of the Old Covenant the Church does this weekly, on the day of the Lord's Resurrection.
Since a "grave cause" is needed to excuse one from this obligation it would be a serious or mortal sin to willfully skip Mass on Sunday or a Holy Day of Obligation, as the Church has always taught. Reasons such as the necessity to work to support one's family, child care, personal sickness or the care of the sick, necessary travel etc. would excuse a person on a particular occasions.