Colossians 2:16-17 (NIV)
Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. These are a shadow of the things that were to come; the reality, however, is found in Christ.
There are a number of different places in the New Testament that support the doctrine that for the Christian, Sabbath keeping is no longer a requirement. I have heard it said that the seventh commandment regarding the Sabbath is the only one riot repeated in the New Testament. In the Old Testament, the Sabbath was the only command given as a sign of the covenant between Israel and Jehovah. With all the strict emphasis on keeping this command in the Old Testament, you would think there would be something about keeping it in the New Testament.
In the above verse, we are told not to worry about anyone judging us wrongly on this issue. I've heard it said that Sunday is the Christian's Sabbath. No, Sunday is simply a day we set aside to honor the Lord in assembling ourselves together. In this sense, Sunday is a holy day and should be treated as such, but it is not the foreshadowing of the Sabbath rest we have in Christ like it was in the Old Testament. Why maintain a substitute when we now have the real thing? As a New Testament Christian, every day is a Sabbath rest, because we rest in Christ's finished work on the cross.