The whole reason for first day assembling is because Jesus rose from the dead on Sunday. Every Lord’s Day is resurrection day, even if we do have a Resurrection Day proper once a year.
If there was motivation to assemble some other day than the seventh, why didn’t Christians go back one, to the sixth, to the day of Jesus’ crucifixion? We preach Christ and Him crucified. It is the word of the cross that is the power of God to salvation. It shouldn’t be difficult to imagine our weeks revolving around every Good Friday.
And while the death of Jesus is explicitly mentioned as what we “proclaim” at the Lord’s supper (1 Corinthians 11:26), and the cross is the place where the Lord Jesus bore our iniquities, He is not on the cross, nor is He ever going back. The cross is empty. The grave is empty. The Lamb stands as having been slain.
We aren’t, we can’t, we won’t forget the Lamb’s sacrifice. The day of His death is necessary and glorious. But if Christ is not risen, our faith is in vain. And Christ is risen, which means the Father accepted His substitution and has no reason to keep believers in suspense. “There is therefore now no condemnation.” Justified by faith, “we have peace with God through our Lord Jesus Christ.” We have been reconciled. The first day is the day of His life.
It is why weekly communion of the He-is-risen kind, is not a burden or boring, but a blessing that feeds our faith and hope and love.