Genesis 2:3

"Then God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done."  Genesis 2:3  NIV

So God is now making the 7th day holy because it is the day he ended his labors of creating the universe.  The QBible translates the word "blessed" as "an act of giving benefit to," so God gave the 7th day a benefit that the other days did not have.  He sanctified it, setting it apart from the other 6 days.

The question is, if God made the 7th day holy, how should we treat the 7th day?  The Jews believed they could not do any form of work that day and the 7th Day Adventists hold that as their day of worship.  

This article argues in favor of keeping the Sabbath as the Jews did, however while they make some good points there are some flaws with their reasoning.  Number 28 under "40 facts about the 1st day" states "the word Sunday never occurs in the Bible at all."  I just have to point out neither does the word Saturday.  That's a silly argument, because these were not the names of the days the Jews would have used, so of course neither of these words would appear in the Bible.  Because of the silliness of this argument, it makes it difficult for me take seriously the rest of the article.  If you have to resort to one argument like this, how can I be sure there aren't more like this?

This article argues that no one day has to be better than any other day.  Paul stated in Romans 14:5&6 that some thought certain days were more holy than others and other thought all days were alike, but neither should judge the other over it because both should be honoring God.  

The first article does make a good point that the Sabbath was set apart long before the Old Law was given.  The second article points out that while the Jewish Christians kept the Sabbath, they did not command the Gentile Christians to.