Typically, when discussing the structure of an English (or Shakespearean) sonnet, we talk about its three quatrains (sets of four lines) followed by a final, rhyming couplet. The rhyme scheme for the English sonnet is usually written out as abab cdcd efef gg (where the "a" lines have end rhymes, the "b" lines have end rhymes, and so on). Italian (or Petrarchan) sonnets, in contrast, are often described as being comprised of an octave (a set of eight lines) and a sestet (a set of six lines); the octave's rhyme scheme is usually ababab or abbaabba, and the sestet can be cdcdcd or cdecde or cdeedc, for example.
That being said, only the first line of this sonnet contains a rhetorical question. Its answer is implied by the seven lines which follow. The person addressed by the narrator is actually "more lovely" and "more temperate" than a summer day, and so the comparison of this person to a summer day is, in some ways, not an apt one. However, her "eternal summer" is better than an actual one. Perhaps the narrator has had other lovers whose passion burned "too hot" (and then lust faded) or whose "gold complexion" became "dimm'd" after a while and the narrator found himself no longer attracted to her. Perhaps another lover was like May's "Rough winds" (and they fought a lot). Just as the person addressed is like a metaphorical "eternal summer," we might read these lines as metaphors for other lovers who were less lovely and temperate than she.