It is one of the Christian ten commandments.
In the Bible, Moses collects ten commandments (refer to Exodus chapter 20). These are supposed to be kept by Christians. One of these commandments is to never work on the sabbath (which for Christians is Sunday but for other relligions Saturday). The ten commandment for Christians is filled with the most important rules to keep and ignoring the sabbath rule is breaking the fourth commandment.
In the Bible, both Jesus and Paul say the sabbath is not required to be kept.
Paul says not to let any man judge you in your observance of Sabbath days. (see Col. 2:16). He also says that these are a shadow of things to come, but the reality is Christ. Furthermore, in Matthew 15:19, Jesus makes a list of the things which defile a man, lining up with nine of the Ten Commandments, but not the Sabbath.
Actually, it is one of the Jewish Ten Commandments.
It is made quite clear in the New Testament that the Old Testament law is no longer required to be kept. I am sure every Christian here agrees with this, because if they did not, they would be sacrificing animals. Therefore, it logically follows that if Old Testament law need not be kept, and a law appears only in the Old Testament, it is unnecessary to keep it.