Katherine Arden Burdine:
(born 1987).
Best known by her pen name Katherine Arden, is an American novelist. Known primarily for her Winternight trilogy of fantasy novels, which are set in medieval Russia and have garnered nominations for Hugo and Locus Awards, she is also the author of the Small Spaces series of horror novels for middle grade children. The first in the latter series, Small Spaces, won the Vermont Golden Dome Book Award in 2020.

You don’t waste October sunshine. Soon the old autumn sun would bed down in cloud blankets, and there would be weeks of gray rain before it finally decided to snow.

The first sentence is a saying about ‘making the most of good weather while it lasts’.
The second sentence gives the reason:
‘October sunshine is a short-lived gift’.
Soon the sun will “bed down in cloud blankets” (the sun is personified as going to sleep under cloud cover), and weeks of gray rain will follow before the first snow. It captures a New England autumn:
‘A brief golden window before the long, dreary stretch toward winter.’ The sentiment echoes older writing about autumn sunshine, such as Nathaniel Hawthorne’s often-quoted remark that he couldn’t bear to waste something so precious by staying indoors, though I can’t say whether Arden meant it as an allusion.
Origin:
The line comes from “Small Spaces” (2018), the first book in Arden’s middle-grade horror trilogy, which is set in rural Vermont in autumn. It appears early in the story and sets the mood: the cozy, fleeting autumn light contrasts with the dread the book builds as it goes on. The only source I found is a quote post that credits the book, so I haven’t checked it against the text itself.
Author: Katherine Arden.
She is best known for the “Winternight” trilogy (“The Bear and the Nightingale”, “The Girl in the Tower”, “The Winter of the Witch”), and the seasonal landscape writing is typical of her style.