I go by the weather, but I'm old fashioned and still abide some of the " not after Labor Day" rules. So in September, I'll wear lighter weight fabrics but not light/bright colors. Maybe no short sleeve or sleeveless. Then, in October, I might start to wear my wool things, in November I'll start to wear cashmere and turtlenecks, also velvet, and that's how ill dress until March, when I'll put away the darker more winter-y things in my closet and go back to what I wore in September. I live in New England, it's cold through march and then it rains until June! I try to dress for the actual weather, while keeping in mind the calendar.