What day does John mean by “the Lord’s day”?

"In Revelation 1:10, John says, “I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet.”

The question is, what day does John mean by “the Lord’s day”?

We believe that there is no clear use of that phrase in any writings of the fathers until near the end of the second century. And if that be

true, the argument for Sunday based on John's use of the phrase stretches out so thin—for it must stretch out over nearly a century—that it cannot carry the weight of argument suspended on it.
Because in the writings of a Second-Century father the phrase “Lord's day” meant Sunday, it does not therefore follow that in the writings of John the phrase meant Sunday.

John wrote the Revelation about the year AD. 90. Up to that time had the Bible writers ever used the term “Lord's day” to describe Sunday? No. They uniformly described Sunday simply as “the first day of the week.” (Acts 20:7; 1 Cor. 16:2; Mark 16:9; Luke 24:1; Mat. 28:1) Even more striking is the fact that John himself, in his Gospel calls Sunday by the same colorless phrase as the other Bible writers used, “the first day of the week.” (John 20:1, 19)

There is only one day described in the Bible that could lay claim to being the Lord's day, and that is the Sabbath. The Ten Commandments describes it as “the Sabbath of the Lord.” Ex. 20:10. Isaiah tells us to call this day “the holy of the Lord.” Isa. 58:13." 

F.D. Nichols