Rashi on Genesis 1:7/רש"י על בראשית א׳:ז׳

1ויעש אלהים את הרקיע AND GOD MADE THE EXPANSE — He put it in proper condition in its place: this is the meaning of “making” it. Similarly (Deuteronomy 21:12) ועשתה את צפרניה “And she shall let grow (literally, make) her nails”.

2מעל לרקיע ABOVE THE EXPANSE — It is not said here על הרקיע “upon the firmament״, but מעל “hanging from above”, because they (the waters) were suspended in space (Genesis Rabbah 4:3). Why is it not stated in reference to the work of the second day “that it was good”? Because the work associated with water was not completed until the third day — He only began it on the second — and anything that is not completed is not in a state of perfection and at its best (and so cannot be termed “good”). Therefore on the third day when He completed the work associated with water and another work was commenced and finished, the words כי טוב are repeated, once in reference to the completion of the work of the second day, and again in reference to the completion of the work of that day (Genesis Rabbah 4:10).