The Greek word mia is the feminine form of heis (one). It is used numerically and can also mean 'first' in the sense of 'on one day' or 'the first day.' Crucially, the resurrection accounts use mia ton sabbaton — 'the first day of the week' — to mark the day of Jesus's resurrection, the foundation of Christian Sunday worship.
The phrase mia ton sabbaton ('the first of the week,' literally 'the one of the Sabbaths') appears in all four Gospel resurrection accounts (Matthew 28:1; Mark 16:2; Luke 24:1; John 20:1). This linguistic marker transformed the seventh-day Sabbath into the first-day celebration of resurrection. The early church met on this day (Acts 20:7; 1 Corinthians 16:2), and it became 'the Lord's Day' — the new creation calendar day that echoes both the first day of creation and the first day of new creation.