I agrees with "Susanna Lives" comments below as to how it came to be that most Christians keep Sunday rather than Saturday. I would like to add a few more in response to others who have responded that the 7th day was for the jews only or that the new covenant does away with keeping Sabbath or that the day was changed to the 1st day.
Why do so many Christians claim that keeping the 4th commandment is irrelevant or that it doesn't apply to NT believers yet they say the other 9 are to be kept. After all, would any good Christian think it is ok to commit adultery or kill? Yet they claim the 4th Commandment doesn't apply or that it was changed, or it is legalistic to keep. Is it possible that since the Sabbath as the 7th day exists since creation (read Genesis), not since Moses went up the mountain, that the reason God starts this, and only this, commandment with the word "remember" is because the Israelites had forgotten while in Egyptian captivity to keep the Sabbath and that God knew at some point, Christians would also forget?
If the actual day doesn't matter, then why at both creation and in the 4th commandment did He specify which day we are to keep? If Jesus changed the Sabbath in the NT, why didn't He tell the disciples but instead He said "For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven." Matt 5:18-19
Also note that the gospel writers clearly outline that the day Christ resurrected was on the first day and all Christians acknowledge Easter Sunday in honor of the resurrection so it is agreed that the resurrection was on Sunday. That said, this leaves that Christ was resting in the tomb on the previous day, Saturday or the 7th day, and here we see that even in His death, Christ rested on the Sabbath. Matt 28:1 says "In the end of the sabbath, as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre". Something as important as a change to one of the 10 commandments would surely have been mentioned in the scriptures and yet it is not.
The new covenant of the NT is that Christ has been sacrificed for our sins and therefore the sacrificial system of the OT is no longer required. This is evidenced by God tearing the curtain to the most holy place of the temple when Christ died (Matt 27:51). When you actually read Col 2:14 is says "having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross." The 10 commandments were not handwritten by Moses but rather by the finger of God. The handwritten laws were the ceremonial laws of the OT which were done away with at the cross.
We do not get to ignore the importance of obedience simply because we are saved by grace. Grace gives us the possibility of eternal life but we must accept that gift and show God our appreciation of the gift by doing His will. Read the epistles of John if you think that obedience is not required of Christians. Even Revelation 14 tells us that the saints "keep the commandments of God".
If you wonder whether keeping the 7th Day Sabbath matters, simply ask yourself whether doing what God asks of us is important. It isn't legalistic to not murder and it's not legalistic to honor God on the day He asks. Remember that what Eve did was in reality a small sin in that she simply ate a piece of fruit from a tree God said not to. Eating fruit isn't wrong, not doing what God asks is wrong.
If you are sincerely looking for answers to the Sabbath question and the history of how it changed to Sunday and why it matters, please go to www.SabbathTruth.com.
May God bless as you read His word and remember to pray that the Holy Spirit interpret it, not the traditions of men.