Why can't you wear white after Labor Day?
Good question. Back in the day, wearing white after Labor day, or before Memorial Day was considered fashion taboo. The rule became more of an etiquette thing with regard to shoes rather than clothing.
Acting Director of the Museum at the Fashion Institute of Technology, Valerie Steele, said in the 1950s, more people were entering the middle classes. These newly-wealthy people were often unaware of the standards of high society, so they were given specific rules to follow in order to fit in. Basically, rules were made so that these uncultured people would wear summer attire in the summer months, and winter attire in the winter months - so as to not embarrass themselves.
Today, of course, the rules are different. I still have to side with the no white shoes after Labor Day rule - because it just looks ridiculous. But, wear your white jeans, jacket etc. all year long! Just be sure that the fabric is seasonally appropriate.