Isn't there an etiquette rule about not wearing sandals after Labor Day?
Sandals can be worn any time of year. Obviously, if it's raining, snowing, or very cold, then sandals would not be appropriate because they do not protect the feet well enough. But, otherwise, if a person chooses to wear sandals in the Fall or Winter, it's up to them. In addition, strappy dressy sandals go well with evening dresses all year 'round.