A good rule of thumb is to change over to your winter tires when the average daily temperature is below 10°C.Snow tires are made of special rubber and have tread designs suited to cold weather. At 7°C they begin to outperform all season tires, but rounding it to 10°C ensures you don’t get caught in an early snow storm.
It’s always best to put on four winter tires. Winter tires are not only there to get you moving, they are there to help to stop. Trying to stop quickly with grippy tires on one end and slippy tires on the other (especially the rear) can easily result in spinning out. Check out this series of videos by Transport Canada on the benefits of using four snow tires.
If you don’t have winter tires, talk to someone who has.They make a difference!