We don't think it's a coincidence that "To Autumn" mentions autumn and spring, but not winter. Keats doesn't want to dwell on the cold days to come. To appreciate autumn, we need to forget about how each passing day seems a little shorter and chillier. For the most part, the speaker stays focused on the present moment, just like the personified figure of autumn, who doesn't seem to have a care in the world. Nonetheless, the poem moves forward in subtle ways. The natural world is at the peak of sunlight and ripeness in the first stanza, and by the third stanza the sun is setting.
The primary tension in the poem is between the forward motion of the day and the season and the speaker's desire to freeze time in each stanza.
The speaker recognizes that autumn has no chance of competing with spring.