Which day is really the Sabbath?
The first Sabbath was the day of rest that God observed after the creation of the world. Genesis 2:2-3 says, "By the seventh day God had finished the work he had been doing; so on the seventh day he rested from all his work. And God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done."
Then, in Exodus 20:8-11, it is recorded that the Israelites were required, in the Ten Commandments, to observe the Sabbath, or seventh day, and keep it holy. This was to be done to represent the covenant that God had made with Israel. The Israelites were to rest from their work and would suffer death if they did not obey this law.
Numerous mentions in the New Testament show that Christ-followers began to honor the first day of the week, Sunday, in observation of the resurrection. So we see that the Sabbath was changed from the seventh day of the week to the first day because of Christ's resurrection. It is a celebration of the new covenant in Christ Jesus.
Some Christians, maybe most, attend church on Sunday. They generally see that day as the Lord's Day, but do not put themselves under a "Sabbath" Law. Colossians 2:16-17 says, "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."
Again, we have an example of how the Law was imperfect, but Christ became the perfect fulfillment of the Law. We are to honor God with our whole lives, not just on one day. We are to celebrate His resurrection continuously, not just on Sunday.