There’s often a day between summer and autumn in Austria, where the weather turns, and everyone knows that at that point things are going to go downhill (certainly in terms of temperature). Winter is coming.
It’s not always a hard and fast rule. After a couple of weeks in the low-teens, it’s not unheard of for temperatures to pop back up to 18-20.
But by then it’s too late.
Austrians have changed their wardrobes.