The difference between a cozy autumn and an autumnal one is that the latter has a more gloomy and somber feel to it. It's a time of change, where the leaves are starting to fall from the trees and people are preparing for winter. There is no one-size-fits-all definition for what constitutes an autumnal one. It depends on who you ask, what their personal definition of an autumn is, or what they're feeling when they see the leaves changing colors. Some people might find this season relaxing while others find it stressful because there's so much to do before winter comes along.