453. How does one keep Sunday holy? Christians keep Sunday and other days of obligation holy by participating in the Eucharist of the Lord and by refraining from those activities which impede the worship of God and disturb the joy proper to the day of the Lord or the necessary relaxation of mind and body. Activities are allowed on the Sabbath which are bound up with family needs or with important social service, provided that they do not lead to habits prejudicial to the holiness of Sunday, to family life and to health. (Compendium of the Catechism of the Catholic Church)

2181. The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin. (emphasis added) Catechism of the Catholic Church. Note: Grave sin means mortal sin and Christians should not receive Communion in mortal sin which is sacrilege. Rather they should approach the sacrament of Confession to confess the sin. All mortal sins must be confessed by number and kind.