Straying a little from his letters (but not far), here are some of Tolkien’s best words about the only season that does not die.

Day 1. Tolkien, writing on the importance of seasons in LOTR:
The main action begins in autumn and passes through winter to a brilliant spring: this is basic to the purport and tone of the tale.
Day 2. Tolkien, writing to his son Christopher in April 1944:
A lovely morning dawned on us this morn. A mist like early Sept. with a pearl-button sun…that soon changed into serene blue, with the silver light of spring on flower and leaf. Leaves are out: the white-grey of the quince, the grey-green of young apple, the full green of hawthorn, the tassels of flower even on the sluggard poplars.
Day 3. 
In The Hobbit, after the Battle of Five Armies:
All the valley had become till again and rich, and the desolation was now filled with birds and blossoms in spring and fruit and feasting in autumn.
Day 4. Before Frodo sets off with the Ring in FOTR:
The sun was warm, and the wind was in the South. Everything looked fresh, and the new green of Spring was shimmering in the fields and on the tips of the trees’ fingers.
Day 5. Frodo singing of Goldberry:
O spring-time and summer-time, and spring again after!
O wind on the waterfall, and the leaves’ laughter!
Day 6. Elvish song which Treebeard sings in TTT:
When Spring unfolds the beechen leaf, and sap is in the bough;
When light is on the wild-wood stream, and wind is on the brow;
When stride is long, and breath is deep, and keen the mountain-air,
Come back to me! Come back to me, and say my land is fair!
Day 7. Sam, waking up after the destruction of Sauron in ROTK:
I feel like spring after winter, and sun on the leaves; and like trumpets and harps and all the songs I have ever heard!