Revelation 1:10 says
"I was in the Spirit on the Lord’s Day, and I heard behind me a loud
voice, as of a trumpet,"
The Lord's Day is defined as the seventh-day Sabbath in Isaiah 58:13-14
“If you turn away your foot from the Sabbath, From doing your pleasure
on My holy day, And call the Sabbath a delight, The holy day of the
LORD honorable, And shall honor Him, not doing your own ways, Nor
finding your own pleasure, Nor speaking your own words, then you shall
delight yourself in the LORD
Jesus, the apostles and even Gentile Christians observed the seventh-day Sabbath for a few hundred years after the resurrection (Acts 13:42, Luke 23:55-56, Matthew 28:1, http://www.sabbathtruth.com/ ) This is best proved by Hebrews 4:9 which says
"There remains therefore a rest for the people of God."
The word rest there is sabbatismos which means a keeping of the Sabbath - http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4520&t=NKJV
The day of the Lord in the Hebrew Bible sometimes refers to a coming judgment which did not occur on the day that John wrote Revelation...
Behold, the day of the LORD comes, Cruel, with both wrath and fierce
anger, To lay the land desolate; And He will destroy its sinners from
it. For the stars of heaven and their constellations Will not give
their light; The sun will be darkened in its going forth, And the moon
will not cause its light to shine. “I will punish the world for its
evil, And the wicked for their iniquity; I will halt the arrogance of
the proud, And will lay low the haughtiness of the terrible. (Isaiah