"Remember the sabbath-day to keep it holy.  Six days shall you labor, and do all your work: but the seventh day is the sabbath of the Lord your God: in it you shall not do any work, you, nor your son, nor your daughter, your servant, nor your maidservant, nor your cattle, nor your stranger that is within your gates: for in six days the Lord made heaven and earth, the sea, and all that is in them, and rested the seventh day: wherefore the Lord blessed the sabbath day and made it holy."

The fourth commandment requires of all men the sanctifying or keeping holy to God such set times as he has appointed in his Word, expressly one whole day in seven.  The Sabbath is to be sanctified by an holy resting all that day, not only from such works as are at all times sinful, but even from such worldly employments and recreations as are on other days lawful; and making it our delight to spend the whole time (except so much of it as is to be taken up in works of necessity and mercy) in the public and private exercises of God's worship.  And, to that end, we are to prepare our hearts, and with such foresight, diligence, and moderation to dispose and to dispatch our worldly business that we may be the more free and fit for the duties of that day. (taken from the Larger Catechism of the Presbyterian Church - Answer to Questions 116-7)

Works permitted by Christ even though they involved "everyday" labor were "works of necessity" (Matthew 12: 1-5) and "works of mercy" (Matthew 12: 9-13).  In this context Christ pronounced that "the sabbath was made for man, and not man for the sabbath: Therefore the Son of man is Lord also of the sabbath." (Mark 2: 27-28)

The sins forbidden in the fourth commandment are all omissions of the duties required, all careless, negligent, and unprofitable performing of them, and being weary of them; all profaning the day by idleness, and doing that which is in itself sinful; and by all needless works, words, and thoughts, about our worldly employments and recreations. (taken from the Larger Catechism of the Presbyterian Church - Answer to Question 119)

Forbidden: Spending His day for self; using it according to your own way, your own will, and for your own pleasure all the while failing to think upon Him and delight in Him, because you are so engrossed with your own interests and concerns and neglectful of the teachings of God's Word on the subject.