There are a few factors that will play a role in the timing of the change from summer types to winter tyres.
Are you expecting harsh winter conditions to arrive? If you are then you have to try to pinpoint when it tends to arrive. Follow the weather forecasts and when the weather starts moving to colder weather and the timing of when you are expecting the winter weather starts approaching, then it is time to book an appointment to have the tyres changed.
If you are very dependent on your car, then you should probably be on the safe side and change a little bit ahead of time, to avoid nasty surprises. The worst you can do is to try to drive to a service station with summer tyres in winter conditions. This should be avoided at all cost.
Studded or non-studded
If you plan to mount studded tyres, then you need to make sure that you follow the local regulations that concerns winter tyres, when it comes to if they are allowed and what date period you are allowed to have them on your vehicle. With the non-studded it is easier and the change can be done earlier.