Why did Moshe need to instruct them to eat the second Omer on Shabbos?
Rashi: Confused at the change in their regimen, on Shabbos morning they asked Moshe whether they should go out to collect the Manna, as they had done for the last six days, to which Moshe replied that what was left from yesterday was what they should eat today. 1
Ramban #1 (on Pasuk 23): Even though they had been allowed to leave it over until the morning, having been forbidden to do so throughout the week, they were hesitant to eat it, until Moshe instructed them to do so..
Ramban #2 (citing Ibn Ezra): Refer to 16:23:2:2.
Seforno: Throughout Shabbos, they were permitted to eat what they leftover from Erev Shabbos - something that would not be permitted to do after Shabbos.
And when he saw their concern that the Manna had ceased permanently, he assured them that today (Shabbos) they would not find it in the field, but tomorrow they would (Rashi).
Why did Moshe repeat the word "ha'Yom" three times in this Pasuk?
Seforno: Moshe instructed them 1. to eat the Manna on occasions during Shabbos; 2. that the entire day they were permitted to eat the Manna that remained from yesterday; 1 3. that from now on, they would never find Manna on subsequent Shabbasos.
Gemara Shabbos 117b: To teach us that is obligated to eat three meals on Shabbos (one at night and two during the day.