WHAT IS THE GREEK MEANING OF THE LORD'S DAY?
There seems to be some debate and confusion over which day "the Lords day" is referring to in Revelation 1:10 the meaning of the scripture from Greek to English is translated from the KJV
Revelation 1:10 , εγενόμην εν πνεύματι εν τη κυριακή ημέρα και ήκουσα φωνήν οπίσω μου μεγαλήν ως σάλπιγγος
Revelation 1:10  I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet.
In the Koine Greek, "the Lords day" κυριακή (kyriakós; G2960) ημέρα (hēméra: G2250) simply means the day belonging to the Lord or the Lord's ownership of "the day".
The word "κυριακη" (translit. "kuriake") is an Adjective - Dative - Singular - Feminine. This means it is being used as a 'possessive' as ownership or belonging to ("of", see 1 Corinthians 11:20, "the Lord's supper"), which means the "day" in context belongs to "the Lord". It is literally "the Lord's (belonging to) day". This means, that the "day" in context is uniquely "the Lord's" out of all the 7 days of the week, for the day under consideration is that which exists within the week, as a day which repeats weekly. This is extremely important, as those who incorrectly assume it to mean "the first [day] of the week" in lieu of Jesus' resurrection, cannot get a weekly occurrence out of a one-time event, in fulfillment of typology of the Firstfruit/Wavesheaf in Leviticus 23:9-14, as made known in 1 Corinthians 15:20; 23
WHAT DAY IS THE LORD'S DAY ACCORDING TO SCRIPTURE FROM THE NEW TESTAMENT?
With the above Koine Greek meanings in mind the next question can be considered. What day is "the Lords day" referring to from the scriptures that the Lord specifically claims ownership of?
Matthew 12:8 For THE SON OF MAN IS LORD EVEN OF THE SABBATH DAY
So according to the very words of Jesus in the scriptures "the Lords day" is the Sabbath day. Here we can see Jesus claiming that he is the Lord of the Sabbath day, taking ownership which is the meaning of κυριακή ημέρα (the Lords ownership of the Sabbath day)
John in Revelation 1:10 is referring to Jesus as creator of heaven and earth by calling Jesus the Lord of the Sabbath just as he did in John 1:1-4; 14. The Lord's day is the Sabbath day!
Mark 2:28, Therefore THE SON OF MAN IS LORD ALSO OF THE SABBATH.
Luke 6:5 And he said to them, that THE SON OF MAN IS LORD ALSO OF THE SABBATH.
Once again the Sabbath day is "the Lords day" and Jesus is taking ownership of the Sabbath day meaning he is the creator God of the Sabbath; meaning he is the God of creation and the creator of heaven and earth (Genesis 2:1-3)
WHAT DAY IS THE LORD'S DAY FROM OLD TESTAMENT SCRIPTURE?
We have seen above from the new testament scriptures that Jesus claims to the the Lord of the Sabbath day. That is he claims to be the God of creation and the Lord of the Sabbath claiming ownership of of the Sabbath day which is the meaning of κυριακή ημέρα (the Lords ownership of the day). According to the scriptures of the new testament Jesus is claiming ownership of the Sabbath day.
In the old testament God has also always claimed ownership of the Sabbath day has His...
Isaiah 58:13  If you turn away your foot from the SABBATH, from doing your pleasure on MY HOLY DAY; shall honor him, not doing your own ways, nor finding your own pleasure, nor speaking your own words and call the SABBATH a delight, THE HOLY DAY OF THE LORD honorable; and
Exodus 31:12-18  Then the Lord said to Moses,  Say to the Israelites, You must observe MY SABBATHS (belonging to; ownership to the Lord; or LORD'S DAY). This will be a sign between me and you for the generations to come, so you may know that I am the Lord, who makes you holy.
Leviticus 19:30 You shall keep MY SABBATHS, and reverence my sanctuary: I am the LORD.
Ezekiel 20:12 Moreover also I gave them MY SABBATHS, to be a sign between me and them, that they might know that I am the LORD that sanctify them.
Once again God is c