The reason I go to church and keep Sunday Holy is because it is The Lords day and the day Jesus rose from the grave. It is also New Testament vs old testament and the same reason that I don't sacrifice animals when I sin, because it is no longer necessary since Jesus rose from the grave. Paul talks about the law not being sufficient and rather a shadow of what was to come. Why hold onto old testament practices, the new testament is here.
While we are under the New Covenant and not the Mosaic Covenant, we are nevertheless still under the same God with the same nature and therefore the same Torah for how to act in accordance with His nature (Jeremiah 31:33). For example, God's righteousness is eternal (Psalms 119:142), so any instructions that God has given for how to act in accordance with His righteousness are therefore eternally valid (Psalms 119:160), and if they were to ever change, such as with it becoming righteous to commit adultery, then God's righteousness would not be eternal.
The Bible contains a number of important foreshadows that teach us about the nature of who God and about His plan of redemption. For example, in 1 Corinthians 5:6-8, Paul spoke in regard to how Passover foreshadowed Jesus as being our Passover Lamb, however, instead of concluding that we no longer need to bother guarding it now that he has come, he concluded that we should therefore continue to observe it. Paul said that they are foreshadows of what is to come, so we should live in a way that testifies about the truth of what is to come by continuing to keep God's holy days instead of a way that denies the truth of what is to come.
In Matthew 4:17-23, Christ began his ministry with the Gospel message to repent for the Kingdom of God is at hand, and the Torah is how his audience knew what sin is (Romans 3:20), so repenting from our disobedience to it is an integral part of the Gospel message. Furthermore, Christ set a sinless example of how to walk in obedience to the Torah, and as his followers we are told to follow his example (1 Peter 2:21-22) and that those who are in Christ are obligated to walk in the same way he walked (1 John 2:6). Jesus did not establish the New Covenant until the end of his ministry, which means that everything that he taught prior to that point was in regard to how to obey the Torah as part of the Mosaic Covenant, and he did not establish the New Covenant for the purpose of undermining anything that he spent his ministry teaching, but rather it still involves following the Torah (Jeremiah 31:33). Furthermore, in Titus 2:14, Jesus gave himself to redeem us from all lawlessness and to purify for himself a people of his own possession who are zealous for doing good works, so the way to believe in everything that Jesus accomplished through his ministry and through the cross is by repenting and becoming zealous for doing good works in obedience to the Torah (Acts 21:20).
In Mark 7:6-9, Jesus criticized the group of Pharisees as being hypocrites for setting aside the commands of God in order to establish their own traditions, so there is nothing wrong in someone choosing to worship on Sunday in addition to obeying God's command to keep the 7th day holy, but people should not hypocritically set aside God's commands in order to establish their own tradition. Disobeying God's law in order to honor the resurrection is working at cross purposes to what Jesus gave himself to accomplish. Jesus rose on the Feast of Firstfruits as the firstfruits from the dead, so there is already a day that is rich with relevant symbolism that we can use to honor the resurrection that is in accordance with what God has commanded, so there is no need whatsoever to hypocritically set aside any of God's commands in order to do that. God blessed the 7th day and made it holy, so it is holy to God regardless of whether or not we recognize it as such, so the issue is whether or not we are going to guard what is holy to God or profane it.