In the first stanza of "To Autumn," Keats personifies autumn as one who is friends with the sun. The personified autumn and sun "conspire" on how to bring fruit and vegetation to their most ripe state. It is just before harvest time; the plants are ripe and full. Autumn is in a vibrant state, so vibrant that the bees might "think the warm days will never cease." The notion of mists and "mellow fruitfulness" indicate an early part of the day.
Autumn is directly addressed in the second stanza as "thee." The speaker considers autumn during harvest time. Again personified, the speaker thinks of autumn sitting on a granary floor as the grain is being harvested. Then the speaker considers autumn asleep, made drowsy by the perfume ("fume") of the poppies. Finally, the autumn is watching the apples in a "cyder-press." Since the first stanza gives subtle indications of being early in the day, the second stanza would be midday or afternoon as autumn has spent "hours by hours" watching the harvest, a sense of some time gone by.
After the first stanza of ripeness and the second stanza of the harvest, the speaker tells autumn not to worry about the upcoming winter or the sounds of spring. Even though the end of autumn signals the death of some vegetation and shorter, colder days, autumn's song (sounds) are just as natural as spring's and summers. Interestingly, the speaker encourages autumn to appreciate her (autumn's) sounds in spite of the melancholy symbols that accompany the colder seasons:
While barred clouds bloom the soft-dying day,
And touch the stubble-plains with rosy hue;
Then in a wailful choir the small gnats mourn
Among the river sallows, borne aloft
Or sinking as the light wind lives or dies;
Words like "soft-dying", "wailful", and "mourn" indicate a mourning time: the end of autumn. The end of any season indicates change; since this is the natural state of things, the melancholia is joined with a sense of joy. Even though Keats (the speaker) mourned the end of autumn, he celebrated its sights, smells, and sounds for what they were. As the first stanza symbolized morning and the second stanza signaled midday, the final stanza signifies evening or night with the phrase "soft-dying day." The completion of autumn is analogous to the completion of a day; the natural progression of things.