Matthew 28:1 (NIV): “After the Sabbath, at dawn on the first day of the week, Mary Magdalene and the other Mary went to look at the tomb.”
Mark 16:1–2 (NIV): “When the Sabbath was over, Mary Magdalene, Mary the mother of James, and Salome bought spices so that they might go to anoint Jesus’ body. 2 Very early on the first day of the week, just after sunrise, they were on their way to the tomb…”
Luke 24:1 (NIV): “On the first day of the week, very early in the morning, the women took the spices they had prepared and went to the tomb.”
John 20:1 (NIV): “Early on the first day of the week, while it was still dark, Mary Magdalene went to the tomb…”
I believe that they resurrected the practice of considering Sunday as the first day of the week, based on the Old Testament tradition. Just like New Earth Creationism is in fact a relatively recent phenomenon driven by a renewed interest in literal interpretations of the Bible, that had long since been superseded in Catholic scholarship.
There are also several patristic writings that conceptualise Sunday as the eighth day of creation, i.e. the resurrection perfecting and completing creation (this also suggests that literal seven day creation wasn’t taken that… er… literally back in the day).