The poem contains apostrophe throughout. Arguably, starting with the title, the whole thing is an apostrophe, since it directly addresses autumn (and, at the beginning of each stanza, reminds the reader that it's doing so).
This address is appropriate given that the poem is an ode, a form that typically praises or describes a person, event, or, as in this case, aspect of nature. Because the entire poem addresses autumn, the apostrophe has time to develop. And as the poem moves forward, the apostrophe grows more direct.
Stanza 1 kicks things off by invoking autumn: "Season of mists and mellow fruitfulness." The poem's title, "To Autumn," firmly links this line with the season of changing colors. The descriptive nature of this first address signals to the reader that the rest of the stanza will deal out some vivid descriptions, and it does: immediately following are the "moss'd cottage-trees," swollen gourds, plump hazelnuts, and overflowing "clammy cells," for example. In the first stanza, then, the address to autumn functions as an entry point into the imagery and material of the poem. By introducing autumn evocatively (rather than plainly) as the "Season of mists and mellow fruitfulness," the speaker gives him- or herself license to delve into the lushness of the scene.
This is necessary, as it builds the poem's setting and context, that of a countryside so overflowing with life that its abundance obviously cannot last. Also, the pairing of "mists" and "mellow fruitfulness" points to the complex identity of the character being addressed. Autumn is at once extraordinarily productive and calm-inspiring, but it also lives behind a mist, a veil with ghoulish connotations that suggests an unseen, lurking presence behind the overwhelming beauty.
Stanza 2 develops the ideas presented in stanza 1. It starts with an even more direct address of autumn, a rhetorical question: "Who hath not seen thee oft amid thy store?" The speaker doesn't expect autumn to reply, but the direct question emphasizes the speaker's impression of the season: autumn is a season so intensely physical that the speaker can't help but think of it as a human being. As in stanza 1, the address in stanza 2 slides back into heavy description, but all of it maintains the image of autumn as a person. For example, autumn is seen drowsing among the poppies and gazing "with patient look" into the dribbling cider press. The address in stanza 2 also develops the mysterious characteristics hinted at in stanza 1. If the "mists" conceal death in stanza 1, they have cleared to expose it in stanza 2, where personified autumn, though apparently harmless, lazes beside "half-reap'd" field with its "hook," a tool that deals death.
In stanza 3, the address gets even more direct. "Where are the songs of spring?" and its immediate reiteration ("Ay, Where are they?") are a different kind of question than the one that starts stanza 2, because their answer is not necessarily implied. Therefore, by addressing autumn, the speaker asks the season to account for itself—specifically, for its uniqueness. Of course, autumn can't respond, at least not as a human would. So, in this final moment, the apostrophe rebounds to the speaker. In order to replace "the songs of spring" with the songs of autumn ("thy music too"), the speaker takes it upon him- or herself to pick out the various animal sounds, such as wailing gnats and bleating lambs, that give autumn its own character.