Labor Day. This rule was originally enforced because wearing white shoes was just not practical in winter, but it also served as a reminder that is was time to put away lightweight summer clothes. However, in recent years, the no wearing white rule has been broken.
Wearing white shoes during the fall and winter months does pose a challenge; as they will show the winter wear a lot easier. If you are going to opt for white shoes, try to wear white boots or look for shoes in a more winter white color. Just make sure you are not wearing sandals or summer shoes in white, as they will not match the season at all.