Winter tires use a softer compound and a tread pattern designed to bite into snow and clear water. Summer and all-season tires harden as the temperature drops, and a hard tire grips less, whether or not there is snow on the road.
The usual trigger for changing over is a sustained temperature around 45°F rather than the first snowfall. Changing back in spring matters just as much - winter compound wears quickly in warm weather.
If your winter tires are on their own wheels the changeover is quick. If both sets share one set of wheels, the tires have to be dismounted and remounted each time, which takes longer and adds balancing.