A long, long time ago, before the invention of sweet, sweet air conditioning, one way to beat-the-heat was to wear light-coloured clothing. Unfortunately, the city was a dirty place (coal!), so you only wore light colours if you a) escaped to your country home for the summer, or b) could afford to replace your whitest whites when they were inevitably ruined by city soot. Assuming you simply spent the summer in the countryside, you would have to trade your light-coloured attire for dark-coloured duds upon your return to the city in September, or risk ruining your favourite summer threads. Hence, no white after Labour Day!