Assuming that the first chapter was composed by the Elohistic source
Actually it fits with the language and style of the so-called Priestly (P) source.
But they also must have realized that all light has to have a source, so why not have the sun created before plant life?
Because the author already believed that light existed independent of the main luminaries. Since the darkness was on the face of the watery deep, the banishment of darkness to the temporal abode of "Night" would involve its replacement by light during "Day". I'm not sure if the author is at all concerned about where the light comes from, anymore than where the darkness comes from. Light simply appears at its designated time, presumably on the face of the deep as well.
BTW, the literary structure of the narrative requires a creation of luminaries later than the creation of light. The whole act of creation is placed in a temporal day-night scheme which marks the passage of units of time, and thus the first act had to be the creation of this temporal scheme. However, since this was the first act, there would be no heaven in which to locate luminaries, as heaven was not created until Day 2. The cosmos at this point is nothing but the watery deep and darkness and wind on top of it. Short of placing the sun and moon inside the watery deep, it doesn't make much sense to describe the creation of the luminaries before the division of the waters. The interesting question tho is why God is not depicted as creating the luminaries on Day 3; why did the creation of vegetation have to precede the luminaries? This is again due to the literary structure. The first three days involved the creation of the environment through division, the last three days involved the creation of the inhabitants of the environment. The first day involved the creation of Day and Night; the fourth day populated Day and Night with the luminaries designed to rule over them. The second day involved the creation of Heaven and the two watery deeps; the fifth day poplated Heaven with birds and flying creatures and the ocean with sea creatures. The third day involved the creation of Land and plants; the sixth day involved the creation of land animals and man (given every plant to eat). The literary structure is also dependent on Psalm 104, which was a primary source of the creation narrative: (1) Light, v. 1; (2) Stretching the heavens out and placement of the "waters above", v. 2; (3) Gathering the terrestial watery deep into one place to create land, v. 5-9; (4) Vegetation to be used as food, v. 14-17; (5) Luminaries, v. 19-24; (6) Sea creatures, v. 25; (7) All other creatures (v. 27-30).
Maybe Day 1 was the creation of matter.
Actually, the narrative assumes that there was matter in the beginning (Genesis 1:2), but it was in a chaotic, unordered state. Creation involved a rearranging of matter (if such a concept as "matter" is at all appropriate), so that the heavens, the heavenly waters, land, and the seas were created through dividing what existed in the beginning in different ways.