The phrase "fall day" refers to a day during the season of autumn, where the weather is cool and the leaves on trees change color and fall to the ground. Full definition
Cool fall days call for a cute faux fur vest. (thestyledfox.com)
It definitely makes me think of cool crisp fall days. (stonegableblog.com)
They released this dress for their summer line, and even though it's a lighter denim I think it's actually better suited for fall days because of the sleeve length. (themodestmomblog.com)