Contrary to their theory of why Christians congregate on the Lord's Day, Sunday, the first day of week, it's not because it was altered by the Catholic church. Scripture records the resurrection of Christ reported to Mary Magdalene (and the other Mary) on the first day of the week.
Mat 28:1 Now late on the sabbath day, as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre..
Compare with Luke's gospel.
Luk 24:1 But on the first day of the week, at early dawn, they came unto the tomb, bringing the spices which they had prepared.
Luk 24:2 And they found the stone rolled away from the tomb.
Luk 24:3 And they entered in, and found not the body of the Lord JesusAct 20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight
Also see Paul's instructions to the church at Corinth:
1Co 16:1 Now concerning the collection for the saints, as I gave order to the churches of Galatia, so also do ye..
1Co 16:2 Upon the first day of the week let each one of you lay by him in store, as he may prosper, that no collections be made when I come.