Or HaChaim on Exodus 16:25/אור החיים על שמות ט״ז:כ״ה

1 אכלהו היום כי שבת היום, "eat it to-day for to-day is a Sabbath, etc." Why did Moses have to command the people to eat the manna? Perhaps this was because he had previously told them to leave aside what was left to keep it till the following morning. He had not spelled out to them that they not only could keep it but were also allowed to eat it. Although we do not have Moses on record as having forbidden the consumption of any left-over manna on other days, it seems evident from the verses about the way to treat the manna on the Sabbath that he must have forbidden the left-overs. Perhaps the prohibition of such left-overs stems from the same consideration as the prohibition to consume meat from sacrificial offerings after the time limit allocated to such a sacrifice has expired. Compare Chulin 114 where our sages state that "anything which I have declared as rejected by Me, you must not eat." Moses' emphasis on "eat it to-day," clearly indicates that it was not to be eaten after to-day. Moses added the words "for to-day is a Sabbath," in order that the Israelites should not form the impression that the prohibition to eat yesterday's manna on the morrow also applied to the Sabbath.

2The last words in the verse may also be understood as the rationale for the people not being able to find manna in the field on that day. In other words: "the reason you will not find manna in the field to-morrow is that to-morrow is a Sabbath for G'd." We may have an allusion here to what Rabbi Shimon bar Yochai said in the Zohar second part page 88: "The food supply for the six weekdays descends from the celestial spheres on the Sabbath; this supply then descends further on a daily basis." Moses only excluded the manna from being found on the Sabbath in our world, i.e. בשדה, in the celestial world it could be found on the Sabbath.