Gen 2:2-3 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.
God finished His work in six days and rested on the seventh day, which the Jews call the "sabbath." The word "sabbath" does not mean "seventh" but "rest." So, we take sabbatical (rest) from our work. It is not a reference to a particular "holy day" but it means simply a "holiday"--a day off from work. Did God rest on the seventh day because He was tired? Unlikely because God is almighty and nothing is too hard for Him. So why did He rest on the seventh day? It simply tells us that God works in a rhythm of work and rest, activity and inactivity. God has created the whole universe to function in this cyclical rhythm of activity and inactivity.
Consider the 24 hour day: there is a 12 hour day to work and a 12 hour night when we rest in sleep. In nature too, there are periods of high activity and low e.g. high tide and low tide; high winds and calms, sowing and reaping, etc. When Scripture tells us that God rested from His work, it is meant to teach us how God works--not in continual frenzy but in seasons. God gives us four seasons to demonstrate this divine cycle of activity (summer) and inactivity (winter). To be fully human and divine, God gives us the command to keep the Sabbath so that we may be like God in our work pattern.
Whether God created the world in six literal 24-hour days or whether the word "day" is a metaphorical expression for "age" is not the subject of discussion here. The point is God uses our understanding of "day" as a 24-hour time period to help us know that we are to follow His pattern of six-day activity followed by a 1 day rest. The Sabbath law has been incorporated into the Ten Commandments to ensure that humans do not overwork themselves and lose the sense of balance and rhythm.
That is why Jesus said that the Sabbath is made for man, not man for the Sabbath. The Sabbath law was given as a blessing to us, to help us function at optimal capacity. It was not meant to keep us under legalistic bondage. In some Christian circles, the Sunday has become the equivalence of the Jewish Sabbath. Instead of a holiday, Sunday has become a holy day. So, if Sunday is holy, we cannot go to work (but it's okay to watch television or go to the movie!) So, instead of focusing on rest, we focus on the keeping the day itself holy. We make Sunday different from the other six days whereas the NT does not make such distinction anymore (Read Colossians 2.16-17) . In Muslim countries, Christians work on Sundays but rest on Fridays. If we think of Sabbath as a principle of 1 day rest in 7, it is fine. But for those who have turn Sabbath into a holy Sunday, then it is problematic to be a Christian in such places.
Like all laws of God, keeping the Sabbath rest day is the way to stay mentally and emotionally sane. Those who ignore this divine pattern of work-rest are endangering their own health and will result in burn-out and sometime early grave. Thank God for the Sabbath. Asking if the Sabbath should be Saturday or Sunday is asking the wrong question about the Sabbath. The Sabbath means rest, and so long as we take one day rest after six days' work, we are obeying the divine rhythm.