"And Moses said, 'Let no one leave any of it till morning.' Notwithstanding they did not heed Moses. But some of them left part of it until morning, and it bred worms and stank. And Moses was angry with them. So they gathered it every morning, every man according to his need. And when the sun became hot, it melted" (Exo. 16:19-21).
The Israelites were quick to obey when they agreed with Moses but trusted their own judgment otherwise. Their experience with other foods had led them to believe that manna would not spoil overnight, but they were wrong! Why would any of them keep manna overnight? Was it because they doubted God would provide more for tomorrow? Lack of faith is a perpetual problem for Israel. Also, it is worth observing that there was a proper time to gather manna and if one delayed too long the opportunity literally melted away.
"And so it was, on the sixth day, that they gathered twice as much bread, two omers for each one. And all the rulers of the congregation came and told Moses. Then he said to them, 'This is what the LORD has said, "Tomorrow is a Sabbath rest, a holy Sabbath to the LORD. Bake what you will bake today, and boil what you will boil; and lay up for yourselves all that remains, to be kept until morning."' So they laid it up till morning as Moses commanded; and it did not stink, nor were there any worms in it. Then Moses said, 'Eat that today, for today is a Sabbath to the LORD; today you will not find it in the field. Six days you shall gather it, but on the seventh day, the Sabbath, there will be none" (Exo. 16:22-26).
God, through Moses, provided very specific instructions for the people regarding what they were to do on Saturday (i.e., the seventh day or Sabbath). They were to make all food preparations on Friday so they would not have any work to do on Saturday. There would be no need to gather any food on the Sabbath or cook anything because that work was already completed the day before. No doubt some of the Israelites wondered why the manna would spoil overnight every night of the week except for the day where they were instructed to gather twice as much. The answer is simple: God did not want it to spoil that evening, so it didn't! Following this instruction would take confidence in the word of the LORD, especially for those who had experienced manna going bad earlier that week. Their past sinful experience would make it even harder for them to obey God now (and so it often is with us today).
"Now it happened that some of the people went out on the seventh day to gather, but they found none. And the LORD said to Moses, 'How long do you refuse to keep My commandments and My laws? See! For the LORD has given you the Sabbath; therefore He gives you on the sixth day bread for two days. Let every man remain in his place; let no man go out of his place on the seventh day.' So the people rested on the seventh day" (Exo. 16:27-30).
Some, despite the prohibition from God, went outside to gather manna on the Sabbath, but it wasn't there (as He had foretold). We might be inclined to give the benefit of the doubt here, suggesting perhaps that they merely forgot, but God's rebuke directed at the people through Moses is much too strong to allow that view. The people were to rest on the Sabbath, not gather food or cook. Some of them rebelled that first Saturday, though they would learn obedience.
The chapter concludes with instructions being given for a memorial of manna which would be kept in a pot in the Ark of the Covenant. Some of the information here is not recorded in chronological order, but it is helpful here for the reader. For example, they had not yet even constructed the Ark of the Covenant which the manna pot would be kept in. Furthermore, at this point they had no idea how long they would be eating manna. Due to their lack of faith, it would end up being four decades!