According to some sources, Christians held corporate worship on Sunday in the 1st century. The earliest Biblical example of Christians meeting together on a Sunday for the purpose of "breaking bread" and preaching is cited in the New Testament book The Acts of the Apostles chapter 20 and verse 7 (Acts 20:7). 2nd-century writers such as Justin Martyr attest to the widespread practice of Sunday worship (First Apology, chapter 67), and by 361 AD it had become a mandated weekly occurrence. During the Middle Ages, Sunday worship became associated with Sabbatarian (rest) practices. Some Protestants today (particularly those theologically descended from the Puritans) regard Sunday as Christian Sabbath, a practice known as first-day Sabbatarianism. (Some Christian groups hold that the term "Lord's Day" can only properly refer to seventh-day Sabbath or Saturday.)
Sunday was also known in patristic writings as the eighth day.
The phrase the "Lord's Day" appears only once in the Bible in Revelation 1:10 which was written near the end of the first century. It is the English translation of the koine Greek kyriake hemera. The adjective kyriake ("Lord's") often elided its noun, as in the neuter kyriakon for "Lord's ", the predecessor of the word "church"; the noun was to be supplied by context.
In Rev. 1:10, the apostle John, used kyriake hemera ("I was in the Spirit on the Lord's Day") in a way apparently familiar to his readers. Observers of first-day worship hold that this means he was worshiping on Sunday, resurrection day. Seventh-day Sabbatarians hold that since Jesus said he was "Lord of the Sabbath" and that Isaiah called the Sabbath the "Lord's Holy Day" then the Lord's Day is the Seventh-day Sabbath (i.e. Saturday). Both parties accordingly use this verse to lay claim to the name "Lord's Day" for their day of worship.mark 2:28
The New Testament also uses the phrase te mia ton sabbaton ("the first day of the week") both for the early morning (Mary Magdalene John 20:1) and evening (the disciples in John 2:19) of Resurrection Sunday, as well as for the breaking of bread at Troas (Acts 20:7) and the day for the collection at Corinth (1 Co 16:2).
The term "Lord's" appears in The Teaching of the Twelve Apostles or Didache, a document dated between 70 and 120. Didache 14:1a is translated by Roberts as, "But every Lord's day gather yourselves together, and break bread, and give thanksgiving"; another translation begins, "On the Lord's own day". The first clause in Greek, "κατά κυριακήν δέ κυρίου", literally means "On the Lord's of the Lord", a unique and unexplained double possessive, and translators supply the elided noun, e.g., "day" (ἡμέρα hemera), "commandment" (from the immediately prior verse 13:7), or "doctrine". This is one of two early extrabiblical Christian uses of "κυριακήν" where it does not clearly refer to Sunday because textual readings have given rise to questions of proper translation. Breaking bread (daily or weekly) may refer to Christian fellowship, agape feasts, or Eucharist (cf. Acts 2:42, 20:7). Didache 14 was apparently understood by the writers of the Didascalia and Apostolic Constitutions as a reference to Sunday worship.
Around 110 AD, St. Ignatius of Antioch used "Lord's" in a passage of his letter to the Magnesians. Ambiguity arises due to textual variants. The only extant Greek manuscript of the letter, the Codex Mediceo-Laurentianus, reads, "If, then, those who had walked in ancient practices attained unto newness of hope, no longer observing Sabbath, but living according to the Lord's life ..." (kata kyriaken zoen zontes). A medieval Latin translation indicates an alternate textual reading of kata kyriaken zontes, informing Roberts's translation, "no longer observing the Sabbath, but living in the observance of the Lord's ".