why does the Season "Autumn' call the"Seasons of mists and mellow fruitfulness" in john Keats'Ode To Autumn?

In Keats' typical "sensuous" poetry, he uses descriptions to highlight the important characteristics of the season. In this ode, "To Autumn" he is commenting on the fact that autumn is a time of harvest (fruitfulness) when the crops are coming to fruition. The use of the adjective "mellow" indicates that the hurry of the spring and summer (the planting, the growing, the nurturing) are now slowing down as autumn moves to winter. The mists are simply another characteristic of the time of the year. What is also important in this ode is that it is an "apostrophe" to autumn rather than a comment "on" some natural such as one might find in some of his other odes.