Rashi on Exodus 16:26/רש"י על שמות ט״ז:כ״ו

1וביום השביעי שבת means but on the seventh day is a day of rest (supply the word הוא); and לא יהיה בו means the Manna shall not be on it; and this verse comes (is written) only to include the Day of Atonement and the Festivals [as being also days when no Manna would fall] (Mekhilta).