I was going to write this long, exceedingly interesting, exceedingly engaging post about the history of the "no white after Labor Day rule," but, as it turns out, it's an exceedingly uninteresting story. Especially considering the lasting power it's had over fashion.
The "no white after Labor Day" rule was just customary for early 20th century wealthier East Coast families (who assumedly set style trends), as they put away their resort clothes after the official end of summer, Labor Day. Being able to afford a separate wardrobe - typically filled with lighter, whiter colors - for summering in their assorted country, beach and mountain homes, and white being a practical color for hotter months, wealthy individuals had no use for their summer whites once back in the city. Further, those whites would easily be trashed in the dirty cities during Fall, Winter and Spring months. Thus, no white after Labor Day was just customary. Where the custom turned into a rule, however, no one seems to know.
So, of course, you're allowed to wear white after Labor Day (and black in the Summer and to weddings). In warmer places like Texas, Vogue says it's "only practical." And, of course, I will wear white year-round, regardless of the rule.