Why do Christians worship on Sunday instead of the Sabbath?
The Christian community as a whole does not keep the Sabbath because it was a part of the legal requirements of the Law, and according to the New Testament the Christian today is not under such legal requirements like circumcision, Sabbath keeping, even tithing. Instead, we are to produce the righteousness of the Law, the character of Christ, through the Spirit (see Rom. 6:14; 7:1-8:13; 2 Cor. 3:4-18; Gal. 3-5).
Further, there are no commandments or exhortations in the New Testament for us to keep the Sabbath. Rather, the early church gathered on the first day of the week in celebration and remembrance of the resurrection. This is evident throughout the book of Acts (cf. also 1 Cor. 16:2 and Acts 20:7). Acts 20:7 is the clearest verse in the New Testament which indicates that Sunday was the normal meeting day of the apostolic church. Paul stayed in Troas for seven days (v. 6) and the church met on the first day of the week.
Note also in the 1 Cor. 16:1f passage that, though Paul is dealing with giving, there is no mention of tithing just as there is no call to assemble on the Sabbath (Saturday) as in the Old Testament. Paul never used the word “tithe” when he discussed giving, even though he gave more attention to giving than any other New Testament writer. Giving should be a systematic, weekly practice on Sunday, the first day of the week, when the church meets together. Giving was also to be proportionate—in keeping with one’s income (cf. Acts 11:29) and like meeting together for worship, Bible study, etc., it was to be the product of the work of the Spirit and not a legal requirement of the Law.