[BJ@Tirerack.com]

I used to change mine out over Thanksgiving break but this year we had quite a bit of snow prior to that. If you're using a true summer tire, you may need to put them on before the snow actually hits so that you don't get stuck in the snow with little to no traction at all. If you're using an all-season for the summer time, you can be a bit more flexible as those will work to get you home in light snow/ice.

The time to change really will depend on your actual location and forecast that year. The key is to play it safe when you use the summer compound tires. It's best to keep those out of the freezing temperatures as well. A day or two in the cold won't ruin the tire but you want to avoid any prolonged exposure whenever possible. (under 40F or so)