Pope John Paul II relies heavily on the witness of the Gospels to flesh out his examination of the Resurrection. Naturally:
20. According to the common witness of the Gospels, the Resurrection of Jesus Christ from the dead took place on “the first day after the Sabbath” (Mk 16:2,9; Lk 24:1; Jn 20:1). On the same day, the Risen Lord appeared to the two disciples of Emmaus (cf. Lk 24:13-35) and to the eleven Apostles gathered together (cf. Lk 24:36; Jn 20:19). A week later — as the Gospel of John recounts (cf. 20:26) — the disciples were gathered together once again, when Jesus appeared to them and made himself known to Thomas by showing him the signs of his Passion. The day of Pentecost — the first day of the eighth week after the Jewish Passover (cf. Acts 2:1), when the promise made by Jesus to the Apostles after the Resurrection was fulfilled by the outpouring of the Holy Spirit (cf. Lk 24:49; Acts 1:4-5) — also fell on a Sunday. This was the day of the first proclamation and the first baptisms: Peter announced to the assembled crowd that Christ was risen and “those who received his word were baptized” (Acts 2:41). This was the epiphany of the Church, revealed as the people into which are gathered in unity, beyond all their differences, the scattered children of God.
The Resurrection was key to the early proclamation of Christ, and so the Resurrection-Sunday is ancient, early, and very much part of the initial Christian vector to observe the Creator’s Sabbath on Sunday rather than Saturday.