Which phrase from john keats's "to autumn" contributes to a mood of serene stillness?

"The prevailing mood in 'To Autumn' is that of the union between joy and melancholy. In my opinion, this poem outlines the theme that joy can only be appreciated in juxtaposition with sadness. Life can only be lived to its fullest extent if death is present at its very conception. The beauty and joy experienced in 'To Autumn' are heightened by the passage of time and the coming of winter.The beauty and joy of the dying day are reflected in and complemented by images evoking sadness:the sun setting on the stubble fields and the wail of the gnats. Contentment which directly evokes sadness and implies acceptance of the process toward death beyond grief, is mirrored in Keats' poem."-skampani | Student, Undergraduate