Genesis 1:8

ויקרא אלוקים לרקיע שמים, as I have already explained previously, the rakia is referred to as shamayim throughout the Torah, whereas the celestial regions in the spiritual sense are called shm’ey hashamyim, “the upper heavens.” Compare Nechemyah 9,6 where we are told:אתה עשית את השמים, שמי השמים וכל צבאם, “it is You Who have made the sky, the heavens and all their hosts.” In Deuteronomy 10,14 Moses wrote: והן לה' אלוקיך שמים ושמי השמים, “both the skies and the heavens are Yours, O Lord.”

ויהי ערב ויהי בקר יום שני, the day faded towards evening until the second morning appeared. Thereby the second of the six days of G’d’s creative activity had come to a close. With this morning the third “day” commenced.

ויקרא אלוקים לרקיע שמים. Seeing that activities occurring in the celestial regions reach us by means of G’d’s agents and not directly, the Torah had to tell us that G’d named the phenomenon, otherwise we would not have known this, [would have considered it as a result of an evolutionary process. Ed.] We know of this indirect activity of G’d when the Torah referred (verses 17-18) to “G’d placed them (the luminaries) in the רקיע השמים, and assigned to them the task to illuminate the earth, etc.”

ויקרא אלוקים לרקיע שמים, after the rakia had become firm, substantial, and it was capable of supporting the luminaries, G’d called it שמים, “heaven.” The name is appropriate, seeing that the rakia was now capable of absorbing luminaries just as heaven is reputed capable of receiving the creatures from the “lower” regions who possess something originating in the “higher” regions, i.e. their souls. One reason why these regions are called שמים is the fact that the luminaries are visible in the sky, heavens.
Our sages, (Bereshit Rabbah 4,7) when commenting on these words, quote Rav as saying that the word שמים means a region in which fire and water coexist. Rabbi Acha son of Kahane, quoting Rav, says G’d took fire and water and thoroughly mixed the two, the result being shamayim. This is the reason why, as we mentioned already, the words כי טוב that it was good, do not appear in the report of G’d’s creative activity on the second “day.” Our sages, (Bereshit Rabbah 4,6) also say that seeing G’d’s activity on the second “day” was not completed on that day, the day’s work did not qualify for the description “it was good.” In order to compensate for this omission, we find the words וירא אלוקים כי טוב, “G’d saw that it was good,” twice in connection with His activity on the third “day.” The first such mention refers to the completion of G’d’s activity involving the waters, whereas the second mention “that it was good,” refers to the earth producing vegetation, the only activity which occurred on the third “day,” according to Ibn Ezra.
Maimonides (Moreh, Kapach edition page 235) writes that the reason why the words “that it was good” do not appear in the report of G’d’s activities on the second “day,” is that because the rakia and all that is above it is part of the “waters,” and is called by that name. It is something concealed from most people, and how could one apply the words “it was good” to something that cannot seen by us to be good and useful? Seeing that the congregating of the waters to a single location, thus making the earth beneath appear could be observed universally, the expression “it was good” was withheld until the report about that stage of the creation. Even though the making of the rakia was the underlying purpose which made vegetation on earth feasible, something which is also concealed from us, the result of the achievement of this purpose was universally visible so that it qualifies as being described by the words “it was good.”