Are you talking about Mia ton Sabbaton (μιᾷ τῶν σαββάτων)? That is basically the day after the Sabbath, that would be the 1st day of the week. It is a description, not a name. However that day is CALLED "κυριακῇ ἡμέρᾳ".
Yes μιᾷ τῶν σαββάτων (First day of the week). This was the weekly day numbering system of the Jewish culture in the OLD and NEW Testament. There was no Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday during this time period in Jewish culture or scripture.
μιᾷ τῶν σαββάτων (First day of the week) was never called κυριακῇ ἡμέρᾳ in any NT scripture anywhere in NT GREEK as the word Sunday was never a naming convention in Jewish culture.
The week days were numbered around the Sabbath for example; First day (μιᾷ τῶν σαββάτων; our Sunday; You will not find Sunday in the NEW TESTAMENT), Second day, Third day, Fourth day, Fifth day, Preparation day, Sabbath.
κυριακῇ ἡμέρᾳ means "Lord's Day" or the day the Lord's has ownership of. This is linked to Revelations 1:10 with John being in the Spirit of the
Lord's Day. This reference of Revelations 1:10 and the Lord's day is not linked to
μιᾷ τῶν σαββάτων (FIRST DAY OF THE WEEK; latter Sunday) anywhere in the NEW TESTAMENT GREEK scriptures.
Yet we see the NEW TESTAMENT scripture of MATTHEW 12:8 links the "Lord's Day"
κυριακῇ ἡμέρᾳ directly to the Sabbath day.
MATTHEW 12:8 [8] For
the Son of man is Lord even of the sabbath day.
κυριακῇ ἡμέρᾳ directly means Lord's Day or day belonging to the Lord or Jesus
Jesus is the creator of the Sabbath where he rested after creating all things in Heaven and earth (John 1:1-3; Psalms 33:6; Colossians 1:16; 1 Corinthinas 8:6; John 1:10; Hebrews 1:2; Ephesians 3:9; Genesis 2:1-3). Jesus made the Sabbath for all mankind and set it aside as a HOLY day of rest for his people (Genesis 2:1-3; Exodus 20:8-11) hence "Lord day"
Looking at the scripture and the Jewish weekly day system it is very clear that
κυριακῇ ἡμέρᾳ (The Lord's Day), scripture alone defines as the Sabbath day (Matthew 12:8). There is not one scripture reference linking
κυριακῇ ἡμέρᾳ (The Lord's Day) to
μιᾷ τῶν σαββάτων (First day of the week) which is the weekly day counting convention of the Jewish weekly system in all the New Testament scriptures.
This is why I asked you ealier to link Κυριακή (The Lord's Day) μιᾷ τῶν σαββάτων (First day of the week) in scripture because I knew you would not be able to as there are no scriptures linking κυριακῇ ἡμέρᾳ (The Lord's Day) to μιᾷ τῶν σαββάτων (First day of the week).
The GREEK words for Sunday, Monday, Tesday, Wednesday, Thursday, Friday, Saturday were never used in the OLD and NEW TESTAMENT scriptures only day one
μία (First day) through to day 6, preparation day and Sabbath day (Seventh day). In the Jewish culture these name descriptions or conventions (day 1, day 2, day 3... preparation day and Sabbath) were what the names of the weekly days were called in NEW TESTAMENT times as all the week days revolved around the Sabbath and this was the week day naming convention.
There is no link to Sunday in the NEW Testament scriptures as Sunday is not there because this was a name convention never used in the Jewish weekly day counting system, only
μία (First day) is used (the weekly day descriptions revolving around the Sabbath). So the word Sunday in Greek is not relevant in reference to NEW TESTAMENT GREEK and scripture as it was never used in NT GREEK scripture.
I have more on this if your interested..
Hope this helps.