Hi, We say both but mostly Fall. Autumn is more of a fancy term but we do use it. I mean in this sentence saying autumn sounds better because a mild fall sounds like you actually fell. Was it a mild autumn last year?

In Britain, we only use Autumn. You could say "It was mild last autumn" or "Last autumn was mild". Erica is right - if you said "Did you have a mild fall last year?" it might mean "did you fall over last year but not hurt yourself much?" No, I had a serious fall and broke both my legs :-)

Maybe "Was the weather mild last fall?" or "Was last fall mild?" if the context is clear. Like Erica said, Americans usually say "fall" but "autumn" is okay too.