Why Did Jesus Rise on a Sunday?

Gnosticism is the heresy that says bodies don't matter and Jesus didn't really rise from the dead physically. That's utterly rejected by the New Testament writers. The fact that Jesus' resurrection is a material physical resurrection is why Christians believe that the material universe matters. Sunday was the first day of the new creation that's now continuing. When we are resurrected and give a new body, the earth itself will be resurrected. But the continuity is important there. It's the same body of Jesus. We will have the same bodies. And so what we do in this world that is in conformity to God's character and his kingdom would endure for eternity because they're good.