Revelation 1:10 - The Lord's Day and the First Day of the Week

Did the apostles observe, or keep, the first day of the week as a Holy Day? Did John in Revelation 1:10 refer to it as "the Lord's Day"?

We personally see nothing in the scriptures that indicate that the first-century Christians were observing Sunday as either a sabbath or "the Lord's Day". It certainly should not considered as an essential doctrine to believe such.

There are some references in the Bible to the first day of the week, but nothing is said about any special observance of that day, either of it being a day of rest, or of it being appointed a day of remembrance of Jesus' resurrection, nor is there any reference to any general custom of observing the first day of the week. None of the scriptures refer to the first day of the week as being "Lord's day." -- Matthew 28:1; Mark 16:2,9; Luke 24:1; John 10:1; 20:19; Acts 20:7; 1 Corinthians 16:2.

John stated: "I was in [Strong's #1722, instrumental "in", by means of] the Spirit on [Strong's #1722, locative "in"], the Lord's [Strong's #2960] day" (Revelation 1:10), and yet there is no indication in those words that he was speaking of the first day of the week. Being "in", or "by means of the spirit" located in time on the "Lord's Day", we believe, means that John had been taken by means of God's Holy Spirit into the future, into the the "last day" (John 6:39,40,44,54; 11:24; 12:48), so as to see the events to be described in the Revelation from that perspective. The Greek word "Lord" here is transliterated with form Kuriakos, which is a form of Kurios. Kuriakos here in the Greek is not indefinite, and it thus evidently not being used to replace the Holy Name, as in "the day of Jehovah." It could be rendered as "day of the Lord." The form Kuriakos only occurs in one other scripture: 1 Corinthians 11:20. It means "belonging to or related to the Lord." It is evidently referring the Millennial Age as corresponding to the "day of Christ." -- Philippians 1:10; 2:16; 2 Thessanlonians 2:2
https://www.studylight.org/lexicons/greek/1722.html.
https://www.studylight.org/lexicons/greek/2960.html

One has to go beyond what is written into later writings to connect an expression "the Lord's Day" to the first day of the week. The Bible never reveals the first day of the week to be "the Lord's Day". It is not for us, however, to speak evil or to condemn any who wish to think of the first day of the week as being "the Lord's day." Let each be persuaded in his own mind. -- Romans 14:5.