Remember the Sabbath and keep it holy
The Sabbath, is the day of resting. The commandments instructs believers to follow the Lord's example and labor for six days, and rest on the seventh. The complete text of the commandment is more lengthy and says:
"Remember the Sabbath day, to keep it holy. Six days you shall labor, and do all your work, 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. 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." - Exodus 20:8 through 11
According to the book of Genesis, God created the world, and everything in it in six days, and then rested on the seventh. This testament is based on this understanding of the world's creation.