“Season of mists and mellow fruitfulness” is taken from -

Conjuring up wonderful images of the delights of Autumn, this line opens To Autumn by John Keats (1795-1821), inspired by a walk the poet took near Winchester on an autumnal evening in 1819. In three stanzas, this exceptionally popular poem describes different characteristics of autumn.
Info Taken from: Oxford Royale Academy

This is also an example of Apostrophe.
Apostrophe: An address to someone absent or something abstract as if the person or the thing were present.
''Season of mists and mellow fruitfulness, Close bosom-friend of the maturing sun''.
Here, the speaker addresses the personified ''Autumn'' throughout the poem. It is an example of an expanded apostrophe.
Source: An ABC of English Literature - Dr. M Mofizar Rahman.