Boyd’s Bible Dictionary:
First day of the week; resurrection day of Christ
(Rev. 1:10). Sunday, after A. D. 321.
Concise Bible Dictionary:
This occurs only in Revelation 1:10: John was in [the] Spirit on the Lord’s day. It was the day of the week on which the Lord arose—the resurrection day. It is the first day of the week, and is suggestive of the beginning of a new order of things, altogether distinct from that connected with the legal Sabbath. It was the day on which the disciples commonly came together for the express purpose of breaking bread, Acts 20:7; and though no legal enactment is given concerning it, it is a day specially regarded by Christians. It is literally “the dominical-day,” κυριακός a word that occurs only in reference to “the Lord’s supper” in 1 Corinthians 11:20 and to “the Lord’s day”; the term is not to be confounded with “the day of the Lord.”