The Catechism is normally the best place to start in looking things like this up when we are unsure. Here is a link to the section on the Sunday obligation:
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2176 The celebration of Sunday observes the moral commandment inscribed by nature in the human heart to render to God an outward, visible, public, and regular worship "as a sign of his universal beneficence to all."109 Sunday worship fulfills the moral command of the Old Covenant, taking up its rhythm and spirit in the weekly celebration of the Creator and Redeemer of his people.
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The Sunday Eucharist
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2177 The Sunday celebration of the Lord’s Day and his Eucharist is at the heart of the Church’s life. "Sunday is the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost holy day of obligation in the universal Church."110
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2178 This practice of the Christian assembly dates from the beginnings of the apostolic age.112 The Letter to the Hebrews reminds the faithful "not to neglect to meet together, as is the habit of some, but to encourage one another."113
Tradition preserves the memory of an ever-timely exhortation: Come to Church early, approach the Lord, and confess your sins, repent in prayer. . . . Be present at the sacred and divine liturgy, conclude its prayer and do not leave before the dismissal. . . . We have often said: "This day is given to you for prayer and rest. This is the day that the Lord has made, let us rejoice and be glad in it."114
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2179 "A parish is a definite community of the Christian faithful established on a stable basis within a particular church; the pastoral care of the parish is entrusted to a pastor as its own shepherd under the authority of the diocesan bishop."115 It is the place where all the faithful can be gathered together for the Sunday celebration of the Eucharist. The parish initiates the Christian people into the ordinary expression of the liturgical life: it gathers them together in this celebration; it teaches Christ’s saving doctrine; it practices the charity of the Lord in good works and brotherly love:
You cannot pray at home as at church, where there is a great multitude, where exclamations are cried out to God as from one great heart, and where there is something more: the union of minds, the accord of souls, the bond of charity, the prayers of the priests.116
The Sunday obligation
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2180 The precept of the Church specifies the law of the Lord more precisely: "On Sundays and other holy days of obligation the faithful are bound to participate in the Mass."117 "The precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the preceding day."118
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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.119 Those who deliberately fail in this obligation commit a grave sin.
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2182 Participation in the communal celebration of the Sunday Eucharist is a testimony of belonging and of being faithful to Christ and to his Church. The faithful give witness by this to their communion in faith and charity. Together they testify to God’s holiness and their hope of salvation. They strengthen one another under the guidance of the Holy Spirit.
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2183 "If because of lack of a sacred minister or for other grave cause participation in the celebration of the Eucharist is impossible, it is specially recommended that the faithful take part in the Liturgy of the Word if it is celebrated in the parish church or in another sacred place according to the prescriptions of the diocesan bishop, or engage in prayer for an appropriate amount of time personally or in a family or, as occasion offers, in groups of families
