The commoner translation of this Hebrew expression (השׁמים ואת הארץ) is “the heavens and the earth”.
Anyway, the comparison between the identical terms שׁמים - inside the verses 1, 6-8 – triggers the conclusion that even if we translate there “the heavens and the earth” this expression must be understood to an ‘expression level’ (terms’ group), not through a word-for-word understanding.
What could be the clues to conclude so?
According the traditional view, the term translated “heavens” (שׁמים) appears in the Hebrew text always in the plural form, as it were prevented to appear in a singular (grammatical) number. For an example, Strong’s Exhaustive Concordance states that this term is a “dual of an unused singular shameh {shaw-meh’}; from an unused root […].”
‘Unused’ root? Unused ‘singular’? Why someone had mint a verbal root (along with the derivative singular of it) if it – all things considered – would be result useless to the subsequent Hebrew speakers?
So, the conviction that ‘heaven’ (singular form of ‘heavens’) cannot appear originally in Bible text is very dubious. Indeed, it is impossible that in all the global Hebrew text (of the Bible) we cannot come across cases in which the singular number (‘heaven’) would be a more apt term respect to the plural number (‘heavens’) of this word.
Think about this, too.
A common Hebrew idiom (in the Bible text) is “X [a given singular term] of Y [a plural form of the same term]”, as we may see – only some examples - in ‘servant of servants’ (עבדים עבד) – Gen 9:25; ‘Holy of Holies’ (קדשׁים קדשׁ) – Exo 26:33, etc.; ‘the song of songs’ (שׁירים[-ה] שׁיר) – Son 1:1; ‘king of kings’ (מלכים מלך) – Eze 26:7.
As you see clearly – even you do not read Hebrew – the two repeated term are identical, except the fact that the second term has the plural termination -im (ים-). In other words, the first term of all the idioms above mentioned are singular words, the second terms are plural forms of the first terms.
This is correct also by logical reasons. In fact, each of these expressions (above their peculiar syntax) have in common the idea of a math set that includes elements possessing identical characteristics (in the examples above, the set of the servants, the set of the holy [places], the set of the songs, the set of the kings). But, among all the elements of a given set there is a single element who stands out in sharp relief against all other elements of the set. Since this is a logical structure, it remain the same also in language different from Hebrew (for a single example, we may remind βασιλευς βασιλεων [‘king of kings’] in Rev 17:14).
In 1 Kin 8:27 we found the expression שׁמים(-ה)שׁמי, including the term at issue. Even though the term at issue – in this verse – appears (in Hebrew) always in a plural form, some translators, understanding the logical structure of the idiom, translate it with a ‘singular > plural’ sequence, as it must be (bold is mine):
“But will God indeed dwell on the earth? behold, the heaven and heaven of heavens cannot contain thee; how much less this house that I have builded?”, KJV, along with ASV, Darby, ERV, Webster, Reina-Valera-Gómez. Even better, the LXX confirms this logical structure: “οὐρανὸς τοῦ οὐρανοῦ”, ‘heaven of heavens’.
As regards Gen 1:1, is there a possibility that שׁמים was originally intended to be a singular noun? We cannot discard this hypothesis. Again, the comparison between the שׁמים of verse 1 compared to the שׁמים of the verse 8 (as we have said above) drives us to conclude that this last שׁמים (v. 8) must be understood in a single-term way, whereas the שׁמים of the v. 1 must be understood in a group-term way, as an idiom.
But, as an idiom, what could be the meaning of it?
Let some commentators speak (the comments without a full reference can be easily found in BibleHub [in a free-way], or consulting the given version itself). As usual, the bold is mine:
Albert Barnes: “[This verse] asserts the creation of the heavens and the earth; that is, of the universe of mind and matter. […] This sentence [of] Gen 1:1 assumes the being of God, and asserts the beginning of things. Hence, it intimates that the existence of God is more immediately patent to the reason of man than the creation of the universe.”
Alberto Canen: “[…] by integrating ‘heavens and earth’ [the Bible writer] attempts to cover everything, all that exist [made of matter]. […] It is also possible that […] because in the Hebrew language there is no word that corresponds exactly to that idea [that is, the Greek kosmos], he [the Bible writer] uses this redundancy of ‘heavens and earth.’” [The Observer of Genesis, the Science Behind the Creation Story, 2014]
John Nelson Darby: “The fact is stated that God created all things, all man sees, all the material universe. ‘In the beginning God created the heavens and the earth’.”
ESV [Gen 1:1 footnote]: “Heavens and the earth […] The text indicates that God created everything in the universe. […] God […] is the Creator of all things that exist.”
Matthew Henry: “[…] the heaven and the earth, that is, the world, including the whole frame and furniture of the universe, the world and all things therein, Act 17:24.”
Jamieson, Fausset & Brown: “the heaven and the earth – the universe.”
Keil&Delitzsch: “’In the beginning God created the heaven and the earth.’ […] This sentence, which stands at the head of the records of revelation, is not a mere heading, nor a summary of the history of the creation, but a declaration of the primeval act of God, by which the universe was called into being.”
Giovanni Luzzi (Italian translation): “I cieli e la terra: modo ebraico per dire l’universo.” [“The heavens and the earth: a Hebrew mood to indicate universe”]
New Jerusalem Bible: “‘Heavens and earth’ are the ordered universe, the result of creation.”
Moreover, it seems to me that the quotation you (Jack Douglas) made of NET Bible footnote can be placed along the lines of the above commentators.
A final gem. Also Sumerians had a term describing the universe: AN.KI. It was a compound word, with the literal meaning of “heaven + earth”.
Getting back to your question (“Does Gen 1:1 refer to day 1 or the entire 6 days of creation?”) we may conclude that Gen 1:1 could have been phrased as a hyper-synthetical sentence which inform us that the physical creations (non-physical creations were yet performed) of God included the universe (as Keil&Delitzsch had yet noted), namely, the ”heaven[s] and earth”, and afterward, He – through Moses – entered into details, describing what He made, inside every creative periods of time.