This is a poem by Aldous Huxley called 'September'. I found it in a fascinating anthology of modern poetry (as of 1922) called Poems of To-Day, which is full of interesting things by poets well-known and long-forgotten. You can read the whole thing online at that link. 'Many of the selections are from the writings of younger men', says the preface; Huxley in 1922 was not yet thirty.
Spring is past and over these many days,
Spring and summer. The leaves of September droop,
Yellowing and all but dead on the patient trees.
Nor is there any hope in me. I walk
Slowly homeward. Night is as empty and dark
Behind my eyes as it is dark without
And empty round about me and over me.
Spring is past and over these many days;
But, looking up, suddenly I see
Leaves in the upthrown light of a street lamp shine
Clear and luminous, young and so transparent,
They seem but the coloured foam of air, green fire,
No more than the scarce embodied thoughts of leaves;
And it is spring within that circle of light.
Oh, magical brightness! the old leaves are made new.
In the mind, too, some coloured accident
Of beauty revives and makes all young again.
A chance light meaninglessly shines and it is spring.