8 "Remember the Sabbath day, to keep it holy.
9 Six days you shall labor, and do all your work,
10 but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates.
11 For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy.
The fourth commandment is central to the structure of most of Western society (and more). Have you ever wondered why a week is seven days long? The lengths of a year, a month, and a day are all astronomical features. That is, we can derive them from the movements in space of the earth and moon in relation to each other and the sun.
There is nothing equivalent for the length of a week. The "Creation Week", in which God worked on the first six days and then rested on the seventh, gives us the seven day week along with the model of resting on one day of the week.
What is the reason for this rest? According to the above passage, it is because God made the heaven and the earth in six days and rested on the seventh - we are to take this day as a remembrance of God's work in Creation. This is a holy day and holy means to be set apart as something different and unique. This is a weekly reminder of the blessings which God has given us.