Optimal temperatures depend on the manufacturer's specifications. For general guidance, 7 celcius is the magic point. Below this, 'normal' rubber tends to become harder. In effect this reduces grip, as the blocks move around less, so generate less heat. Also tyre rubber that is harder loses its ability to conform to the road surface, which also reduces grip.
Winter tyres work fine above 7C, but not as well as 'all season' or 'summer' hoops. Is essence you won't trash your winter tyres by using them in warmer conditions, but bear in mind their wear rate will increase.
Generally I wait until I see regular frosts in the morning, or that old cheerful sight of the first gritter of winter. Then it's wheels & tyres swapped.