Lechem Mishneh: Back To The Beginning VI
“God said to Moses, Behold! I shall rain down for you food from heaven; let the people go out and pick each day’s portion on its day, so that I can test them, whether they will follow My Teaching or not. And it shall be that on the sixth day when they prepare what they will bring, it will be double what they pick every day.” (Exodus 16:4-5)
“When they prepare what they will bring.” Even the collection of Manna, including Friday’s portion, was considered an act of preparation for Shabbat. The week is a preparation for Shabbat, just as this world is preparation for the World To Come, the Day of Eternal Shabbat. Just as Adam’s eating of all the other trees was to be a preparation for eating from the Tree of Knowledge in its proper time.
The two Challot represent the joining of the weekdays to the Shabbat.