#4 "Remember the sabbath day, to keep it holy."
For the Ten Commandments to make any sense and in turn be taken seriously, we must see them as a complete list. "The Decalogue" implies they are a cohesive whole.
So what about observing the Sabbath? Secularists and even some Christians don't even acknowledge the rest of the passage where God says why we should observe it. Whoever wrote the book of Hebrews says it's because we are to "rest from our works, as God did from His" (Hebrews 4:10), implying that the work of keeping the law, in this case "observing the Sabbath", was completed with Jesus' Atonement. With His death and resurrection, we now see that the emphasis on obedience has shifted to an inward observance as opposed to an outward, dogmatic rule-keeping. And with no belief in God as Creator, let alone His very existence, it's understandable why this fourth commandment is easily dismissed. Especially in our free-market, capitalistic society. I should point out, however, that businesses still observing the sabbath by closing their doors on Sunday still do quite well for themselves, in spite of the fact that they're only open six days a week. The Sabbath by the way, is actually Saturday. This is why Seventh-Day Adventists attend church then.
I don't mean to dilute the literal meaning of the Commandment, but consider this: Jesus said that He was "Lord even of the sabbath day." (Matthew 12:8) This implies, to my mind, that He is not only fulfilling the commandment, but superseding it. He was censuring the Pharisees as He said this. They had made the Commandment of "none effect because of [their] tradition" (Matthew 15:6, referring to the fifth, yes—but I see it as the same negative attitude). My opinion is that when we follow a bunch of hard-and-fast rules, we're missing the opportunity to follow Him. And should the Holy Spirit lead you to do something on Saturday, or Sunday, or any other day for that matter, that might conflict with the Fourth Commandment, then don't hesitate! Obey the inner witness of the Spirit. Jesus showed us how. Read Matthew 12:1-8.
"Then said Jesus unto them, I will ask you one thing; Is it lawful on the Sabbath days to good, or to do evil? to save life, or to destroy it?" (Luke 6:9)
Work on that one.