It was Sunday that Christians suddenly started calling, “the Lord's day” [A κuρiaκή ήμέρα].1
• Gospel of Peter 35 -- “[when] the Lord's day dawned, when the soldiers [were guarding Jesus's tomb]…"; 50 -- "…at the dawn of the Lord's Day Mary Magdalene, [came to the tomb]…” (Trans. by Brown).
• Bible: Revelations 1:10 -- “I was in the Spirit on the Lord’s day…”
• Ignatius to the Magnesians 9:1 -- “…no longer observing sabbaths but fashioning their lives after the Lord's day, on which our life also arose through Him” (Trans. by Lightfoot & Harmer. 1891)
• Didache 14 (c. AD 100?) -- “On every Lord's Day—his special day—come together and break bread and give thanks, first confessing your sins so that your sacrifice may be pure.” See discussion in Richard Bauckham's “The Lord's Day,” in From Sabbath to Lord's Day ed. Carson (Zondervan, 1982), 230-2.
This is relevant because
James Dunn (NT professor at Durham): “Sunday had become a day of special significance for Christians, ‘the Lord's day’, precisely because it was the day on which they celebrated the resurrection of the Lord.” [Jesus Remembered (Wm. B. Eerdmans, 2003), 860.]
• Early Christians eschewed Saturday (Sabbath) worship gatherings for Sunday worship gatherings. That is to say, early Christians suddenly stopped gathering for worship on the standard Jewish holy day of rest, opting to meet and worship on Sundays instead.[Forthcoming] This is relevant because Jews were otherwise tenacious about obeying the commandment concerning the Sabbath, and nothing other than their belief that Jesus resurrected on Sunday would warrant this change.
Craig Blomberg (NT professor at Denver): “…something dramatic must have happened on that first Sunday to cause Christians to stop resting and worshiping on the Sabbath, the day commanded by God from the time of the Ten Commandments onward to be set aside as holy, and replace it with Sunday observance (Acts 20:7; 1 Cor. 16:2; Rev. 1:10).” [Jesus and the Gospels (Broadman & Holman, 1997), 353.]. Richard Swinburne (Philosophy professor at Oxford): “There are other days on which it might have been more natural for Christians to celebrate the Eucharist (e.g. on the day of the original Last Supper—probably a Thursday and certainly not a Sunday— or annually rather than weekly). No such are known. There is no plausible origin of the sacredness of Sunday from outside Christianity. There is only one simple explanation of this universal custom, which, I argued, must derive at the latest from the first two or three post-Resurrection years. …[It was because] the central Christian event of the Resurrection occurred on a Sunday. Yet such early practice would have included that of the Eleven themselves, and so could only go with a belief of theirs that Christians had seen either the empty tomb or the risen Jesus on the first Easter Sunday.” [The Resurrection of God Incarnate (Clarendon, 2003), 165.]