Acts of Peter
Around 170 AD, Dionysius, Bishop of Corinth, wrote to the Roman Church, "Today we have kept the Lord's holy day ("kyriake hagia hemera"), on which we have read your letter." In the latter half of the 2nd century, the apocryphal Acts of Peter identify "Dies Domini" (Latin for "Lord's Day") as "the next day after the Sabbath," i.e., Sunday. From the same period of time, the Acts of Paul present St. Paul praying "on the Sabbath as the Lord's Day ("kyriake") drew near."