What is the central idea of the poem "Ode To Autumn" by John Keats?

In 1918, this is what Keats wrote of the poem:

"How beautiful the season is now--How fine the air. A temperate sharpness about it. Really, without joking, chaste weather--Dian skies--I never liked stubble-fields so much as now--Aye better than the chilly green of the spring. Somehow, a stubble-field looks warm--in the same way that some pictures look warm. This struck me so much in my Sunday's walk that I composed upon it."

"Ode To Autumn" is no mystery it is what it is: a poem written in praise of autumn. Autumn in its fullness, autumn in its ripeness, autumn in its satisfying, still warm beauty.

One can certainly think of autumn as but an end to summer and the harbinger of the cold, desolate months of winter. But Keats sees autumn as a fruition, the bountiful result of the promise of rapid growth that began in the spring and went all the way through summer. Autumn, in the poem is summer matured, summer's result.

Listen to this line:

Season of mists and mellow fruitfulness!
Close bosom-friend of the maturing sun

and this:

And full-grown lambs loud bleat from hilly bourn

Keats does not see autumn as a time of withering, a time of fall; he sees it as life at its height.