Maybe you don't have the time or the cash to have regular facials. For you, a facial might be just a yearly indulgence. But if you are going to do it only once a year, the best time just might be autumn.
Although the outdoor cold and indoor dry heat in the winter are punishing to the skin, the worst time for your skin is probably summer. Considering the time spent in the sun, the exposure to chlorine in pools, and the increased time outdoors (which means more contact with air pollution), summertime can be very damaging to the skin. A fall facial might help to repair some of the damage the summer season has caused. It could unclog your pores, get rid of dead skin cells (remember that sunburn?) and moisturize dry skin -- all the better to face the trials of the winter ahead!