The meaning and origin of the expression: Season of mists and mellow fruitfulness

Season of mists and mellow fruitfulness

What's the origin of the phrase 'Season of mists and mellow fruitfulness'?

From John Keats' poem, To Autumn, 1820:

Season of mists and mellow fruitfulness,
Close bosom-friend of the maturing sun;
Conspiring with him how to load and bless
With fruit the vines that round the thatch-eves run;
To bend with apples the moss’d cottage-trees,
And fill all fruit with ripeness to the core;
To swell the gourd, and plump the hazel shells
With a sweet kernel; to set budding more,
And still more, later flowers for the bees,
Until they think warm days will never cease,
For Summer has o’er-brimm’d their clammy cells.

See also, 'Here lies one whose name was writ in water'.