For Christians, the first day of the week. The Greeks and Romans knew of seven planets, and named the days of the week after them: dies solis was one day, as was dies martis, etc. Christians at first used the same names as the Jews, so that the last day of the week was the Sabbath and Friday was the eve of the Sabbath. From the 4th cent. CE the Church adopted the names of the planetary week and the rich symbolism surrounding the sun and its light could be appropriate to the Church which worshipped Christ, the true sun, on ‘the Lord's Day’, which had been the name used in the NT period (Rev. 1: 10). The Easter narratives of the gospels (Mark 16: 2; Matt. 28: 1; Luke 24: 1; John 20: 1) all speak of the ‘first day of the week’. A week later, the eighth day (John 20: 26) was the next Sunday.
Christians first began to worship on Sunday rather than on the Sabbath quite soon. This is suggested by the description of Paul at Troas (Acts 20: 7–12), confirmed by the Didache (14: 1) and by Ignatius. The communities probably met in the evening, following the Easter traditions (e.g. Luke 24: 29).