We all have heard the rule that no white should be worn after Labor Day, but many don't know how this rule started. In the late 19th century, rich, high society women who came from wealth found themselves rubbing elbows with the so-called "new rich." In order to distinguish old money families from the newfound millionaires, they used fashion rules that only those who were "in" would know about. These rules included things like wearing the correct sleeve length for certain events, and, of course, only wearing white in the summertime. Eventually, when Labor Day became a federal holiday in 1894, the holiday became a natural end-point for summer, and for wearing white.
Another common explanation has to do with the availability of air conditioning and methods to keeping cool. In the 20th century, wardrobes were not as casual as they are today. People wore what many would now consider formal clothing. So instead of donning a pair of shorts and a tank top, men and women would wear white and light-colored clothing that was lighter in weight. This was done in an effort to keep cool during the heat of summer. After Labor Day, when summer gave way to fall, wearing light fabrics would leave people too chilly, so people cast aside white for darker, heavier fabrics.
Whatever the reason for this rule, times have changed! Wear what you want and rock it the best way you can.