When God blessed, sanctified, and hallowed the seventh day (Genesis 2:3; Exodus 20:11), it was after He had formed from the dust of the earth the very pinnacle of His physical creation. The physical creation was finished, but God's sanctification of the next day shows that physical man was not complete. The spiritual dimension was missing. God set apart the seventh day to show that His spiritual creation is now taking place.
The Sabbath day is so central to God's purpose that He gave extra instructions to help us fully receive and benefit from this weekly gift. We say that "time is money," but the fact is that time is even more precious than money. If we really have to, we can almost always make more money. However, every person is limited in the amount of time he has, whether in a day, a week, or his life. Because of how precious time is, God wants us to make the most of the Sabbath, that time that is transcendently better than the rest of the week. Thus, in His providence, He gave us the Preparation Day, which sets the stage so that we can properly receive the gift of the Sabbath.
The first mention of the Preparation Day occurs at the same time that the Sabbath was re-taught to the children of Israel in Exodus 16. God commands the Israelites to gather their food for the Sabbath on the sixth day, so there would be as few distractions on the seventh day as possible. The Preparation Day, then, is the day to complete all our cleaning, cooking, business transactions, etc., so that the ordinary and the mundane activities of life do not detract from this time that God has set apart.
With Israel, God was dealing with a carnal people, so His instructions deal primarily with the physical aspects of preparation. Physical preparation for the Sabbath helps us to receive and make proper use of this holy time. Note that God gave this principle to Israel even before the fourth commandment was actually spoken at Mount Sinai.
However, there is spiritual preparation to be done as well, a fact that Israel did not have a heart to understand. They did well just to obey the letter of the law, even nominally. We, though, are called to look at the spirit—the essence—of the principle in addition to applying it physically.
Jesus, in John 6, tells us what the manna actually represents:
Therefore [the people] said to Him, "What sign will You perform then, that we may see it and believe You? What work will You do? Our fathers ate the manna in the desert; as it is written, 'He gave them bread from heaven to eat.'" Then Jesus said to them, "Most assuredly, I say to you, Moses did not give you the bread from heaven, but My Father gives you the true bread from heaven. For the bread of God is He who comes down from heaven and gives life to the world." Then they said to Him, "Lord, give us this bread always." And Jesus said to them, "I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst. . . . I am the bread of life. Your fathers ate the manna in the wilderness, and are dead. This is the bread which comes down from heaven, that one may eat of it and not die. I am the living bread which came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I shall give is My flesh, which I shall give for the life of the world." (John 6:30-35, 48-51)
These verses define the real spiritual manna about which we need to be concerned. It is not only the Bible; it is Christ Himself. True Christians should desire Jesus to dwell within them, not just His words. For us, then, the essence of the instruction in Exodus 16 is to ingest, invite in, or assimilate the Person of Jesus Christ daily. He is the "daily bread" we are told to ask for in the Model Prayer (Matthew 6:11; Luke 11:3).
Thus, manna, the "bread from heaven," is linked to ingesting Jesus Christ to gain spiritual nourishment and sustenance. This is done, not only through Bible study, but also through continual prayer and seeking His involvement throughout the day. In short, eating manna symbolizes strengthening our relationship with God so we can receive His attributes.
God fully intended the Israelites to eat the manna every day, as it was their only staple. But the unique instruction in Exodus 16 deals with gathering twice as much on the Preparation Day. They did not eat twice as much then, but they had to expend twice the effort on that day to ensure that they had enough to eat the next day.
If we look at this spiritually, we, too, have to ingest Jesus Christ every day through prayer, Bible study, and experiencing life with Him. On the sixth day, however, we need to "gather" twice as much, expending twice the effort to ensure that we have enough of the bread from heaven to eat on the Sabbath.