Autumnal comes from the Latin adjective autumnālis, meaning "of or belonging to auntumn", which is derived from the Latin noun autumnus (autumn).  The base word came into Middle English as autumpne via Old French (autompne / automne) in the late 14th century.  The exact origin of the Latin autumnus is uncertain. Linguists trace it as a loanword likely of Etruscan origin rather than a native Indo-European root. Some traditional Roman etymologies linked it to augere ("to increase" or a rich season), though modern philologists view that as folk etymology.  

According to the Oxford English Dictionary (OED), the adjective "autumnal" was first used in English during the Middle English period (1150-1500), with the earliest recorded evidence appearing around 1440 in Palladius’ De Re Rustica.  The evolution of the meaning of autumnal is as follows: 1570s: Used to mean "maturing in autumn", 1630s: Used to mean "belonging to autumn", and 1650s: Adopted in the figurative sense of "past the prime".  

Autumn plays a powerful role in storytelling because it naturally embodies transition, duality, and emotional depth. 

Key Themes of Autumn in Storytelling: 
Transition and Change: Autumn marks the bridge between the vibrant expansion of summer and the quiet dormancy of winter. It is ideal for characters crossing from one state of life, age, or world into another (such as coming-of-age journeys or shifting kingdoms in fantasy). 
Abundance and Harvest: Historically and practically, autumn is the time of the harvest—gathering what was sown earlier in the year. In narratives, this mirrors characters reaping the consequences of their past choices, talents, or hard work. 
Maturity and Aging: If spring represents childhood and summer represents youth, autumn symbolizes adulthood, wisdom, and maturity. It captures a character looking back with reflection rather than forward with blind impulse. 
Beauty and Impermanence: The glowing colors of changing leaves combine the visual splendor of peak vitality with the quiet acceptance of decay and letting go. Writers use this to evoke nostalgia, grief, or romantic melancholy.  
Atmosphere and Mood: Crisp air, mist, early twilight, and cozy indoor rituals naturally heighten tension, mystery, or comforting intimacy, making autumn a staple for folklore, Gothic tales, cozy mysteries, and atmospheric fiction (such as Over the Garden Wall).  

Just the facts:

AUTUMNAL Definition & Meaning - Merriam-Webster

AUTUMNAL in Thesaurus: All Synonyms & Antonyms