You should notice as well that the declaration of "good" varies as to when it is said within a day in Genesis 1 (all references unless otherwise noted are to chapter 1).
Day 1 - it is stated once right after the creation of light (v.4a) while the earth was still without form (v.2-3), but before the dividing of light and dark (v.4b-5).
Day 2 - it is left unstated during the day (as your question notes), a day in which division of water and firmament (or expanse) is made (v.6-8).
Day 3 - it is stated twice: first, after the gathering together of seas and appearing of the dry land (v.9-10); second, after the plants emerged from the earth to reproduce itself (v.11-12), which appears to be at the end of the day (v.13).
Day 4 - it is stated once, after the luminary bodies are formed in the sky to divide the light from the dark in traceable cycles (v.14-18), which appears to be at the end of the day (v.19). NOTE: this completes the division of light and dark from day one, a division that did not merit a "good" statement from God.
Day 5 - it is stated once, after the creation of the sea and air creatures (v.20-21), but prior to the blessing of them (v.22), which blessing occurred at the end of the day (v.23).
Day 6 - it is stated twice, once after the creation of land creatures (v.24-25), and once after the creation of mankind, along with the responsibilities and blessings (v.26-30), at the final culmination of His creative work, at which point the whole is called "very good" (v.31).
It is only upon the completion of elements for the sustaining of human life that the notation of being good is made.
There end up being seven total "completion" segments, which after Jack Douglas posted his observations regarding a pattern of the relationships, I then realized these seven segments formed a specific pattern themselves, and have thus edited my answer along those lines (still keeping my base argument about completion segments). Using the same "was so"/"was good" parallels noticed by Jack, along with my maintaining the key completion statement as being the "was good" statements rather than the end of day statements, then we observe the following.
Segments 1 & 4: Light itself was needed in contrast to the dark for the sustaining of material life on day 1 (v.5; completion segment #1), but the division of light and dark was not good until God made the division understandable to mankind by the creation of the luminous bodies on day 4 (v.18; completion segment #4). The "was [light/]so"1 (v.3 & 15)/"was good" (v.4 & 18) are in each segment in a parallel relation to the corresponding segment.
Segments 2 & 5: The division of heaven, water, and land was not good until the land had appeared, where man would dwell, early day 3 (v.10; completion segment #2). The plant life that would be food for mankind (v.29) was good after it was in place to reproduce itself and be available for man to eat from, late day 3 (v.12; completion segment #5). The "was so" statements are both found in segment #2, and the "was good" are split, one at end of #2 and one at end of #5 (of course, since "was good" is our key for something being completed). Between the corresponding segments, however, the relationship is not directly parallel, but rather a chiastic/parallel relation:
- (A) "was so" (day 2, v.7)
- (a) "was so" (day 3, v.9)
- (a') "was good" (day 3, v.10)
- (A') "was good" (day 5, v.21)
Chiastic in that a/a' completes the bounding of the seas (to create land), whereas A/A' completes the purpose of the seas' creation to house its lifeforms. Parallel, in that the wording is the same (A = a/a' = A').
Segments 3 & 6: The creatures coming out of the waters, over which mankind would have dominion (v.28), were good after they could reproduce themselves, day 5 (v.21; completion segment #3). The creatures coming out of the land, over which mankind would also have dominion (v.28), were good after they could reproduce themselves, early day 6 (v.25; completion segment #6). The "was so"(v.11 & 24)/"was good"(v.12 & 25) are in each segment in a parallel relation to the corresponding segment, just as segments 1 & 4.
Segment 7: Adam was not good alone (Gen 2:18), but once able to have help and reproduce with Eve (Gen 2:22-25), then together they could be fruitful, fill and subdue the earth, and have dominion over the creatures (v.28), at which point creation was culminated by the introduction of the created caretakers, and all could be declared "very good" (v.31; completion segment #7). This statement is both a statement about mankind, but also about all of creation, since mankind was the culmination for which creation was made. This "was so" (v.30)/"was [very] good"(v.31) stand alone as the culminating statements, and have no corresponding parallel to any others.
This factor of various parts of creation coming to completion for the sustaining of God's ultimate creature, His image bearer that is like Him (v.26-27), mankind, is a major argument against gap creation theory, which relies heavily on believing that God could not create anything "without form and void" (KJV, v.2) because such would not be good. Such a view fails to recognize that during the creation days, there were only points at which it was good--between those points, each stage was not yet in place to be habitable by mankind, and so it was not good until each phase was complete.
The declarations of goodness are not intrinsically aligned to the period of time during which the creation is occurring (i.e. the days themselves), but rather tied to the completion of various aspects of creation: (1) Light, (2) Land (as related to Sea and Heavens), (3) Plants, (4) Light and Darkness division for time keeping, (5) Air and Sea animals, (6) Land animals, (7) Mankind as God's image bearer (and totality of creation).
1 Also assuming, along with Jack, that
we are happy to count the slightly anomalous "and there was
light" in day 1 as "and it was so"