Autumnal refers to things that are suggestive of autumn.
Few sights are more beautiful than trees with brilliant foliage—bold reds, yellows, oranges, and browns. It’s pretty clear why the season during which the leaves turn is called fall. After all, the leaves fall from the trees. But we also call this season autumn and describe the changing leaves as autumnal. What are the origins and usage of this term? Let’s start with some quotes.
I cannot endure to waste anything so precious as autumnal sunshine by staying in the house.
—Nathaniel Hawthorne, The American Notebooks
[T]hat old September feeling, left over from school days, of summer passing, vacation nearly done, obligations gathering, books and football in the air . . . Another fall, another turned page: there was something of jubilee in that annual autumnal beginning, as if last year’s mistakes had been wiped clean by summer.
— Wallace Stegner, Angle of Repose
As you might have guessed from the month mentioned in Wallace Stegner’s writing, autumnal refers to things that are suggestive of autumn, a season that takes place during the months of September, October, and November in the Northern Hemisphere. In the Southern Hemisphere, it extends from the March equinox to the solstice in June. Two equinoxes occur every year, and the one that happens in September is sometimes called the autumnal equinox, fall equinox, or September equinox. This event marks the crossing of the sun over the earth’s equator.
A second definition of autumnal refers specifically to humans. It can mean beyond the age of maturity or middle age. Notice how John Donne illustrates this usage in a quote from The Complete Poetry and Selected Prose:
No spring nor summer beauty hath such grace as I have seen in one autumnal face.
Here is another quote from John Clare: The Critical Heritage. Here autumnal refers to the last part of a period of time:
Crabbe calls up, on the whole, an autumnal gloom of childhood, contrasts the things which delighted with the gloom by which the delights were impaired . . . incapable of conceiving a poem . . .
The root word of autumnal (and autumn, for that matter) is unknown, but some theorize that it comes from an Etruscan term. According to the Online Etymology Dictionary, it may come from an archaic word that was associated with the idea of a “drying-up season.”
What are the synonyms of autumnal? A more general term is seasonal, but this could refer to any of the four seasons, depending on the context. You might also refer to fall or the harvest instead of using this adjective.
Whether you are extolling the beauty of the harvest, the crisp air, or a mature human face, you can use autumnal to refer to all things autumn. What’s your favorite aspect of the season? Now you can use autumnal in a sentence to describe it. Now what about the other seasons?
from Grammarly Blog https://www.grammarly.com/blog/what-does-autumnal-really-mean/