It's OK, we don't, but if one wishes to observe a Sabbath Day they are free to do so, that is what Romans 14 is all about!
Paul in writing to the Corinthians acknowledged the early church met on Sunday>
1 Corinthians 16:1-3
King James Version
16 Now concerning the collection for the saints, as I have given order to the churches of Galatia, even so do ye.
2 Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.
3 And when I come, whomsoever ye shall approve by your letters, them will I send to bring your liberality unto Jerusalem.
Paul strongly implies here that the church met on Sunday. But Sunday is not the Sabbath and the church is not bound to the Sabbath. The Sabbath was given to the jews as a sign between them and God!
Exodus 31:13
Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it is a sign between me and you throughout your generations; that ye may know that I am the Lord that doth sanctify you.
also, when the church was all jewish, the believers would stay after Sabbath service and when sundown came (beginning of Sunday) they would hold a Christian worship service.