Autumnal

autumnal \aw-ˈtəm-nəl\ adjective: 1. of or relating to autumn; occuring in autumn, 2. having to do with a period of maturity or incipient decline

Etymology: Middle English autumpne, from Latin autumnus

I had always pronounced this word correctly: auTUMnal. Then I read somewhere that the stress should be placed on the first syllable: AUtumnal. It felt wrong, but I complied. Now Merriam-Webster corroborates my initial instinct. Either I misread that other authority (whose identity is now forgotten) or I dreamed up the whole thing.