I am so pleased that you said "beautiful autumn" day. On the BBC is is autumnal this and autumnal that. I thought I'd never hear the right word ever again.
Autumnal:I think an 'autumn day' is a day in autumn, whereas an 'autumnal day' is a day which shows characteristics of autumn.
Peterborough will disappear beneath the waves
Or a day like a day during autumn.Autumnal:
Of, characteristic of, or occurring in autumn.
https://en.oxforddictionaries.com/definition/autumnal
Ergo: A day occuring in Autumn would be an Autumnal day.
Exactly.Or a day like a day during autumn.
Which could be in Summer.Exactly.
Or even Spring or Winter.Which could be in Summer.
I think an 'autumn day' is a day in autumn, whereas an 'autumnal day' is a day which shows characteristics of autumn.
Fashion item now though. You should see the prices of some.What do we call a day that has characteristics of spring?
What's the difference between "autumn" colours and "autumnal colours."
I may just accept that occasionally there is a need to use "autumnal" but I get the feeling it is just a television/radio fashion to do so. Have a good listen and you'll hear it pop up where the root word could have done just as well. Another fashion that I notice is for presenters to carry branded cue cards that they never use, have mugs of coffee in front of them which they never drink from and my current obsession, presenters walking around with backpacks with (clearly) nothing inside.
This shampoo I just bought is sooo....vernal.