Why Sunday and not Saturday in the New Covenant?

The First-Century Church set the precedent for Sunday worship as God separated Light from darkness on the First-Day; Jesus was raised from the dead by the Spirit on the First-Day; the Holy Spirit initiated the New Covenant Church on the First-Day at Pentecost; the New Covenant is superior to the Old, it’s NEW and mandated a separation from that which is Old and could never provide salvation, only condemnation (Romans 3:20).

6 Now, however, Jesus has received a much more excellent ministry, just as the covenant He mediates is better and is founded on better promises. 7 For if that first covenant had been without fault, no place would have been sought for a second. Hebrews 8