Biblically speaking the matter should be clear without the aid of commentaries. Consider Jesus' own words:
And he said unto them, The sabbath was made for man, and not man for
the sabbath: Therefore the Son of man is Lord also of the sabbath.
(Mark 2:27-28, KJV)
This shows Jesus as having made the claim that he was the Lord of the Sabbath, i.e. that the Sabbath was "the Lord's day." John, who was so close to Jesus, would have remembered this and used it to tell his readers which day it was when the revelation had come to him.
The Sabbath has always been claimed by our Lord as His day.
Speak thou also unto the children of Israel, saying, Verily my
sabbaths ye shall keep: for it is a sign between me and you throughout
your generations; that ye may know that I am the LORD that doth
sanctify you. (Exodus 31:13, KJV)
Ye shall keep my sabbaths, and reverence my sanctuary: I am the LORD.
(Leviticus 19:30 & Leviticus 26:2, KJV)
And this truth is repeated multiple times in the Gospels.
For the Son of man is Lord even of the sabbath day. (Matthew 12:8,
And he said unto them, That the Son of man is Lord also of the
sabbath. (Luke 6:5, KJV)