The early Christians celebrated the Sabbath on Saturday, often attending synagogue and all that. (Though they didn’t have to.)
But Sunday, the day of the Lord’s resurrection, was called the Lord’s Day (Dominicus dies in Latin, Kyriake in Greek). That was the day the early Christians got up very early and went to Mass, because Sunday was a Roman, Greek, and Jewish work day.
It was only much later that Christians began calling Sunday “the Sabbath”. (And sometimes they even called Saturday “the Preparation day” (Paraskeve in Greek), just as Jews called Friday the Day of Preparation, because it was the day before the Sabbath.) In some languages, though, Sunday is called “Resurrection”.
From the earliest times, Christians celebrated Friday as a day of penitence (because that was when Christ died) and Sunday as a day of joy.