Well this describes my mother to a tee, along with men must remove hats indoors and proper young ladies wear gloves, table manners, et al.

Labour Day is considered the end of summer and hence the end of summer fashion. Remember, that in the Victorian and Edwardian eras (Labour Day was established in 1894) the dictates of fashion and ettiquette were everything. They were what kept one apart from the 'commoners'. White also was a sign of wealth as it required a great deal to keep startched, 'blued', and clean.

I admit, having grown up with such rules drilled into me, I find myself cringing when I see someone in white bottoms after labour day.