The first day of the week, sometimes called the eight day of the week, also called the “Lord’s Day,” is set aside for public worship. This was true in the early Church under the Apostles who recognized that the Christian mystery supplanted that of the Old Law, the Sabbath.
On this day, Catholics are obliged by law to assist at the Sacrifice of the Mass.
By grant of the Congregation for the Clergy (January 10, 1970), the faithful, where the bishop considers this a pastoral benefit, may satisfy the Sunday precept of attending the celebration of the Eucharist by participating in the Mass in the late afternoon or evening of the preceding Saturday. The same grant extends to the time before a holy day of obligation.
The following quotations are just two examples to show that the first Christians understood this principle and gathered for worship on Sunday. “But every Lord’s Day . . . gather yourselves together and break bread, and give thanksgiving after having confessed your transgressions, that your sacrifice may be pure. But let no one that is at variance with his fellow come together with you, until they be reconciled, that your sacrifice may not be profaned” (Didache, A.D. 70).
And another, “We keep the eighth day with joyfulness, the day also on which Jesus rose again from the dead” (Letter of Barnabas, A.D. 74). (8:22)