What is the prevailing mood in "To Autumn"?

The prevailing mood of "To Autumn" is peace and contentment. The year is winding down. The fruit trees and vegetables have matured and ripened, the fields have been harvested, the granaries have been filled. Like spring, autumn has its own music:

Then in a wailful choir the small gnats mourn
        Among the river sallows, borne aloft
            Or sinking as the light wind lives or dies;
    And full-grown lambs loud bleat from hilly bourn;
        Hedge-crickets sing; and now with treble soft
        The red-breast whistles from a garden-croft;
           And gathering swallows twitter in the skies.

While there is a peaceful mood about the poem, there is a hint of sadness as well.

I cannot read one of Keats's poems without thinking about how short his life was and how his poetry reflects his thoughts on life and death. He died of tuberculosis at the age of 26 in 1821. He wrote "To Autumn" only two years earlier. In a sense, Keats was in the autumn of his own life. By showing peace and contentment in the closing of the year, Keats was in essence saying that he had come to terms and was at peace with the fact of his illness and imminent death.