I think it depends on how easy it is for you to have your tires changed over.
If you have to go to a garage I use Oct 31-Nov 15 as my rule of thumb. No line ups and friendly service. If you wait until it snows or the day before (by weather forecast) you may experience very long lines/wait times.
If you can do it yourself, anytime after Nov 1 is fine. It probably takes about 1 hour to do all four tires, just a little slower than the Nascar guys.

Don’t forget to use your torque wrench.