It applies most especially to white shoes. One does see ads for "winter whites," that is, white woolen clothing.
Actually, I think the convention - which is indeed of long-standing in the U.S. - applies exclusively to shoes. I give you the leading US etiquette authority of the current era, the puckish Judith Martin (Miss Manners):
“From Memorial Day to Labor Day, you may wear white shoes. Not before and not after. As a command, the White Shoe Edict should be clear and simple enough. Do not violate it. In a society in which everything else has become relative, a matter of how it makes you feel, a question between you and your conscience, and an opportunity for you to be really you, this is an absolute. Miss Manners not only doesn't want any argument about it, she doesn't even want any discussion. For goodness' sake, if you want to smash the conventions of your own heritage, find a rule that is more fun to break.
Considering that she had previously decreed this, once and for all, Miss Manners has been tossing letters she has received on the subject into a box marked "Oh, Stop It." However, it doesn't stop, and the time has come to clean out the box. Once and for all:
Memorial Day is Memorial Day, not Memorial Day Observed. It is on May 30. Confederate Memorial Day has nothing to do with shoes.
Yes, the rule applies to gentlemen as well as ladies. As a matter of fact, even in season, a gentleman never wears white shoes except with sports clothes and during the daytime.
No, sneakers don't count, except on the day they are purchased. After that, they're not white, anyway.
Yes, white shoes that are a part of certain set outfits worn for specific activities may be put on for the duration of that activity. Tennis players and brides and those taking first communion may wear white shoes, but they must change when the respective events are over. (Tennis players should also shower. Brides are always fresh. Miss Manners digresses.)”
This thread reminds me of what writers of dictionary and grammars call the tension between prescriptive and descriptive. Such books tell you what a word means now, what is currently accepted usage, or conventional grammar, etc. But language is constantly changing and there is always a battle between those who want to continue to promote the inherited rules as correct and those who want to change them to reflect the changed reality of daily life. Such debates are how fashion (in language or clothes) is made and we see them playing out on this website and in the world everyday. But as Sator has amply demonstrated the iGents aren't rear-guard protectors of fading traditions, they are the sartorial equivalent of people who make etymological back-formations, or who invent grammar rules that never were and then hector people for not following them. (The kind who correct you for using who instead of whom and get it wrong anyway.)
The iGentry pick and choose from the flotsam of history the bits and pieces of long-ago conventions and fictional pasts that catch their fancy, reassemble them Frankenstein-style into something that never was or could have been, and then wail at our fall from this Neverland paradise. This seems to me to be very much of a piece with a broader social trend of the last few decades. I can think of any number of politicians, social critics, journalists, holiday marketers, and writers who do the same in other topics. ("Ye olde traditional Christmas," anyone?)
Which is why I think it is so refreshing that Sator approaches these debates with such evidentiary rigor. I hold no objection to anyone following whatever set of dress rules they like, or even promoting them to others, but the specious claims of historical authority are just too much Personally, I think Sator is doing a great public service with this mythbusting.