The poem I have in mind is part of The Trees of Kortirion and is called Narquelion, which, in the Quenya language of the Elves, means "sun-fading":
Alalminórë! Green heart of this Isle
Where linger yet the Faithful Companies!
Still undespairing here they slowly file
Down lonely paths with solemn harmonies:
The Fair, the first-born in an elder day,
Immortal Elves, who singing on their way
Of bliss of old and grief, though men forget,
Pass like a wind among the rustling trees,
A wave of bowing grass, and men forget
Their voices calling from a time we do not know,
Their gleaming hair like sunlight long ago.
A wind in the grass! The turning of the year.
A shiver in the reeds beside the stream,
A whisper in the trees - afar they hear,
Piercing the heart of summer's tangled dream,
Chill music that a herald piper plays
Foreseeing winter and the leafless days.
The late flowers trembling on the ruined walls
Already stoop to hear that elven-flute.
Through the wood's sunny aisles and tree-propped halls
Winding amid the green with clear cold note
Like a thin strand of silver glass remote.
The high-tide ebbs, the year will soon be spent;
And all your trees, Kortirion, lament.
At morn the whetstone rang upon the blade,
At eve the grass and golden flowers were laid
To whither, and the meadows bare.
Now dimmed already comes the tardier dawn,
Paler the sunlight fingers creep across the lawn.
The days are passing. Gone like moths the nights
When white wings fluttering danced like satellites
Round tapers in the windless air.
Lammas is gone. The Harvest-moon has waned.
Summer is dying that so briefly reigned.
Now the proud elms at last begin to quail,
Their leaves uncounted tremble and grow pale,
Seeing afar the icy spears
Of winter march to battle with the sun.
When bright All-Hallows fades, their day is done,
And borne on wings of amber wan they fly
In heedless winds beneath the sullen sky,
And fall like dying birds upon the meres.