The 4th Century bishop, Eusebius of Caesarea (d. 339AD), comments on the importance of Sunday to the early Christians.
“’On this day (Sunday) which is that of the light, the first, and also that of the true sun, we gather together after the interval of six days and, celebrating holy and spiritual Sabbaths, we carry out that which He prescribed for the priests to do on the Sabbath according to the spiritual Law….It was on this day that at the time of creation, when God said: Let there be light, there was light; and on this day also the sun of justice arose on our souls (P.G. XXIII, 1172 B).’
Roman Catholic priest, Fr. Jean Danielou, continues:
“It is this same idea of the First Day as being both that of the generation of the Word and that of the Resurrection which Eusebius takes up again in commenting on the verse of the Psalm: ‘You have delighted me with Your works, O Lord, and I rejoice in the works of Your hands.’ He writes: ‘This is the day that the Lord has made, let us rejoice and be glad in it.’ He means the Lord’s Day of Resurrection, as we have shown elsewhere in explaining what concerns the creation of the World. God said: ‘Let there be light, and there was light.’ You see that on this Day there was no other creation for which the word would be fitting: ‘This is the Day that the Lord has made,’ except that Day itself which was the first Lord’s Day. It is of this day that it is said: ‘Lord, You have delighted me with Your works.’ As to the works of His hands, these are they that were created in the days that followed’ (P.G. XXIII, 1173 B-1176 A).” (The Bible and the Liturgy , pp 252-253)
It was on day 1 that God spoke causing light to exist. Many early church fathers understood this “light” not as a physical light, but God speaking His Word into existence. The Word of God is light to the world. Sunday, as the day of resurrection is the new creation, God again calling light into being in the face of darkness. A light which knows no end.