The clearest instance of God's command to keep one day of worship is found within the Ten Commandments in Exodus 20:
Remember the Sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is the Sabbath of the Lord your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. For in six days the Lord made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the Lord blessed the Sabbath day and hallowed it. (verses 8-11)
These four verses contain a wealth of vital principles regarding the Sabbath day, but these five stand out:
1. We are to remember and keep the Sabbath.
2. The seventh day is God's Sabbath.
3. On the seventh-day Sabbath, those who observe it are to do no work, nor should any under his authority or on his premises—human or animal.
4. The Sabbath memorializes the Creation and God's rest on its seventh day.
5. The Sabbath, blessed and hallowed by God, is holy time.
In Deuteronomy 5:12-15, the Sabbath commandment is restated but with a different reason:
And remember that you were a slave in the land of Egypt, and that the Lord your God brought you out from there by a mighty hand and by an outstretched arm; therefore the Lord your God commanded you to keep the Sabbath day. (verse 15)
Moses here adds an addition purpose for the Sabbath day: It is a memorial of our redemption from Egypt, a type the world apart from God. Those whom God has called to be His sons or daughters have been plucked from the world and granted a new life. The Sabbath is designed, God says, to help us to remember our former lives of sin and godlessness as well as all that He has done for us to turn our lives around.
It is not hard to see that the Sabbath is very important to God and to us. If we want to please God, we must "meet with Him" on the day He has set aside for worship.