This is a heresy taught by several different groups and churches. They teach that the church has always supposed to have kept the Sabbath and if you do not and meet on Sunday instead you are the ones committing a heresy, this is a false and unbiblical teaching. The Jews under the Law of Moses were commanded to keep the Sabbath day (Saturday) as a sign of their relationship with God (Exodus 20:8-11, Ezekiel 20:12 & 20, Nehemiah 9:14).
Since Jesus “finished” the Old Testament law we are no longer under the bondage of the law but we are under grace (Romans 6:14 – For sin shall not have dominion over you: for ye are not under the law, but under grace.). The Sabbath is no longer in effect as a sign between God and His people. Colossians 2:16-17 could not be any clearer about this it says “Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ.”
The “first day of the week” (Sunday) is the appointed day of gathering together for meeting and worship for true New Testament Christians. The disciples and the New Testament Christians met and worshiped on the first day of the week (Acts 20:6-7, I Corinthians 16:2). Jesus rose and completed our redemption on the first day of the week (Mark 16:9). Jesus met with the disciples after His resurrection, more than once, on the first day of the week (Mark 16:9-12, Matthew 28:8-10, Luke 24:36). Jesus blessed His disciples and gave them the Holy Spirit on the first day of the week (John 20:19). The Gospel of a risen Savior was first preached on the first day of the week (Acts 2 – we know the Pentecost was on a Sunday because it was always on the 50th day after Passover – Leviticus 23:15-16). Christians do not keep the Sabbath, or they should not, though we know some erring groups do. Christians following the New Testament teaching on the subject meet on the first day of the week, what is called the Lords Day by John in Revelation 1:10, or what we all know as Sunday.