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 claiming ownership of the Sabbath day (Revelation 1:10 κυριακή ημέρα) calling the Sabbath "my holy day" or "the holy day of the Lord" and "My Sabbaths"
Deuteronomy 5:14 BUT THE SEVENTH DAY IS THE SABBATH OF THE LORD YOUR GOD...
Leviticus 23:3 ... THE SABBATH OF THE LORD...
Exodus 31:15 ... THE SABBATH OF REST HOLY TO THE LORD
Exodus 20:10 BUT THE SEVENTH DAY IS THE SABBATH OF THE LORD YOUR GOD...
Once again God is claiming ownership of the Sabbath day (Revelation 1:10 κυριακή ημέρα) to the "Sabbath of the Lord" or "the Sabbath of rest holy to the Lord".
WHY DOES GOD AND JESUS CLAIM OWNERSHIP OF THE SABBATH DAY?
Why then does God and Jesus claim ownership of the Sabbath day (Revelation 1:10 κυριακή ημέρα)?
Genesis 2:1-3 , Thus the heavens and the earth were finished, and all the host of them. , And ON THE SEVENTH DAY God ended his work which he had made; and he RESTED [שׁבת; shâbath H7673; Keep Sabbath] on the SEVENTH DAY from all his work which he had made. , And GOD BLESSED THE SEVENTH DAY AND MADE IT HOLY: because that in it he had RESTED [שׁבת; shâbath H7673; Keep Sabbath] from all his work which God created and made.
Links to God’s 4th Commandment
Exodus 20:8-11 , REMEMBER THE SABBATH DAY, TO KEEP IT HOLY [Made Holy at creation Genesis 2:3]. , Six days shall you labor, and do all your work: , But THE SEVENTH DAY [Genesis 2:1-3] IS THE SABBATH OF THE LORD YOUR GOD : in it you shall not do any work, you, nor your son, nor your daughter, your manservant, nor your maidservant, nor your cattle, nor your stranger that is within your gates: [WHY?] , FOR IN SIX DAYS THE LORD MADE HEAVEN AND EARTH, THE SEA, AND ALL THAT IS IN THEM IS, AND RESTED THE SEVENTH DAY; WHEREFORE THE LORD BLESSED THE SABBATH DAY AND MADE IT HOLY. [Referring to Genesis 2:1-3]
God claims ownership of the Sabbath because on it he rested from creating the heavens and the earth where he blessed the Sabbath as a holy day of rest for all mankind as a memorial of creation and a celebration of God as the creator of Heaven and earth (Mark 2:27-28).
The "Lords day" is the Sabbath day according to the scriptures. There is no scriptural links to "the Lords day" in Revelation 1:10 being in reference to "Sunday" or the "the first day of the week". In Revelations 1:10 "Lords Day κυριακῇ ἡμέρᾳ does not link to any scripture reference in the NEW TESTAMENT to μιά των σαββάτων (the FIRST DAY OF THE WEEK). In Revelation 1:10 κυριακῇ ἡμέρᾳ is used in reference to the Lord's ownership of the day which as shown above from the scriptures from both the old and new testament that the day Jesus and God claims ownership of is the Sabbath day. Revelation 1:10 says no where in the scriptures that this day is in reference to μιά των σαββάτων which means the first day of the week. The claim that "the Lords day" is Sunday is a teaching and tradition of men that is not supported in the scriptures.
CHALLENGE FOR THOSE WHO CLAIM SUNDAY IS THE LORD'S DAY
Your challenge here is that if you disagree with the scriptures provided above that prove that "the Lords day" from Revelation 1:10 is the Sabbath day, please provide a single scripture from God's Word showing that "the Lords day" in Revelation 1:10 is referring to Sunday or the first day of the week. Scripture only please as we know what the man-made teachings and traditions of men are in regards to "the Lords day" from sources outside of the bible. This OP is to examine if these outside sources outside of the bible are biblical or not.