Adam Clarke on Isaiah 30:26

Shall be sevenfold - The text adds כאור שבעת הימים keor shibath haiyamayim, "as the light of seven days, "a manifest gloss, taken in from the margin; it is not in most of the copies of the Septuagint. It interrupts the rhythmical construction, and obscures the sense by a false, or at least an unnecessary, interpretation.

By moon, sun, light, are to be understood the abundance of spiritual and temporal felicity with which God should bless them in the days of the Messiah, which should be sevenfold, i.e. vastly exceed all that they had ever before possessed.