A fashion rule is that one should only wear white (or bright colors) during the summer—from Memorial Day until Labor Day. It’s not known when this “rule” began and many people do not follow it. The logic is that wearing white makes one cooler in summer; wearing black makes one hotter in winter.
 
“Wear white in summer” was cited in print in 1882. “Whatever else you wear, whatever other fashion you prefer, you will wear white this Summer” was cited in 1901. “The debutantes, as a rule, wear white this summer, with large, fluffy hats and white shoes” was written about Newport, Rhode Island, in 1902.
 
“It is in better taste not to wear white after Labor Day” was cited in print in 1947. “The tradition of no-white-after-Labor-Day on feet” was cited in 1949. “You never wear white after Labor Day” was cited in 1955. “Gone are the days when no one would wear white shoes after Labor Day” was cited in 1964.
 
“Don’t worry, it’s okay to drink white after Labor Day” is a jocular variation of the saying.
 
   
Wikipedia: Labor Day
Labor Day in the United States is a holiday celebrated on the first Monday in September. It is a celebration of the American labor movement and is dedicated to the social and economic achievements of workers. It constitutes a yearly national tribute to the contributions workers have made to the strength, prosperity, and well-being of their country.
 
Old Fulton NY Post Cards
16 January 1882, Hudson (NY) Daily Evening Register, pg. 2 (?), col. 3:
FOR THE LADIES.—Wear white in summer. Whether on the street, or at home, or on the beach, it is the color for the season.
 
Chronicling America
29 August 1884, Weekly Graphic (Kirksville, Adair Co, MO), “Coloring Butter,” pg. 4, col. 1:
We array ourselves in clothing of varied hues, although schientists claim we should wear white in summer and black in winter, and they must be cut the latest style or we are not happy.
 
Old Fulton NY Post Cards
5 May 1901, Brooklyn (NY) Daily Eagle, pg. 1, col. 7 ad:
White Fabrics Low.
Whatever else you wear, whatever other fashion you prefer, you will wear white this Summer. Nothing ever takes the place of white for hot weather wear.
(Abraham and Straus.—ed.)
 
Old Fulton NY Post Cards
30 August 1902, New Rochelle (NY) Press, “Summer Chat” (from Newport, RI), pg. 2, col. 3:
The debutantes, as a rule, wear white this summer, with large, fluffy hats and white shoes.
 
Old Fulton NY Post Cards
23 May 1908, Auburn (NY) Citizen, “Fashion Notes” (from McCall’s), pg. 10, col. 5:
It is now considered perfectly correct to wear white in summer, even when in deep mourning.
 
Old Fulton NY Post Cards
11 June 1912, Rochester (NY) Democrat and Chronicle, pg. 17, col. 4 ad:
A White Summer
The expected demand for white shoes materialized in early spring. Now that June has come—the first really big month for wearing white shoes—the call for footwear in white materials has become unusually strong.
 
All conditions are now favorable for the very general spread of this fashion.
(Wm. Eastwood & Son Co.—ed.)
 
Chronicling America
6 August 1912, Washington (DC) Times, pg. 16, col. 6 ad:
White canvas footwear
Practically nine-tenths of the women this summer are wearing white shoes of some description, and a large proportion of the white shoes worn are of white canvas—and women will continue to wear them up until fall—but we must dispose of those we have on hand because it is the time when we look forward to making space for fall goods.
(S. Kann Sons & Co.—ed.)
         
Old Fulton NY Post Cards
23 June 1916, Syracuse (NY) Herald, pg. 4, col. 6 ad:
White Shoes
OF course you will wear white shoes this summer. Trim, smart and fashionable, they add the finishing touch of distinction and coolness to the summer costume.
(Park-Brannock Co.—ed.)
 
Chronicling America
6 June 1918, Daily Kentuckian (Hopkinsville, KY), pg. 8, col. 4 ad:
THE IDEAL—THE PATRIOTIC FOOTWEAR FOR SUMMER IS “WHITE.”
WEAR WHITE SHOES
(Wall & McGowan’s Boot Shop.—ed.)
 
Chronicling America
26 June 1918, Public Ledger (Maysville, KY), pg. 1, col. 5 ad:
Wear White Shoes This Summer
D. Hechinger & Co.
 
Old Fulton NY Post Cards 
17 June 1947, Philadelphia (PA) Inquirer, “Fashion Queries” by Cynthia Cabot, pg. 18, col. 6:
It is in better taste not to wear white after Labor Day.
 
Old Fulton NY Post Cards
12 November 1949, Buffalo (NY) Courier-Express, “Sedate White Is Favorite Colorfor Winter hats” by Alice Hughes, pg. 13, col. 7:
Odd, this break from the tradition of no-white-after-Labor-Day on feet, perhaps another reversion to the Twenties.
 
Google Books
The American Mercury
Volume 80
1955
Pg. 72:
And that’s how I learn that you never wear white after Labor Day, even if you cook.
 
Old Fulton NY Post Cards
6 September 1959, Buffalo (NY) Courier-Express, “Enquiring Reporter: Do You Swith To Fall Clothes on Labor Day?” by Jim Bowback, pg. 7-C, col. 4:
JUDY DUBIL, Kenmore, secretary. (...) “The only thing I follow is no more white clothes or white shoes after Labor Day. That’s one fashion rule I never break.”
 
23 February 1964, Sunday Advocate (Baton Rouge, LA), “Togs to Cruise Into Spring” (NEA), pg. 9-D, col. 3:
Gone are the days when no one would wear white shoes after Labor Day. They can be seen during nearly all seasons today.
 
8 September 1967, Mount Carmel (IL) Daily Republican-Register, “In This Corner” by Shirley Northcote, pg. 2, col. 3:
In fact, as you moved up the ladder at your place of employment, it seemed one of the most important phases of your career, NEVER wear white shoes after Labor Day.
 
4 September 1972, The Plain Dealer (Cleveland, OH), Jeanne Albers column, pg. 7-F, col. 1:
Yikes. It’s Labor Day already.
 
So that means today is the day to start packing away your light, bright summer clothes, right? After all, nobody wears white after Labor Day, right?
 
Wrong. Absolutely, positively wrong. How many times do I have to remind you that those silly rules are long gone by the boards.
 
Google Books
Elsa’s Housebook:
A Woman’s Photojournal
By Elsa Dorfman
Boston, MA: D.R. Godine
1974
Pg. 76:
Never wear white shoes after Labor Day.
 
26 August 1979, The Plain Dealer (Cleveland, OH), “Future is black for those who wear white too late” by Erma Bombeck, sec. 4, pg. 10, col. 1:
I don’t know what happens to people who wear white suits, white dresses or white shoes after Labor Day.
   
Mental Floss
August 30, 2014 - 10:02pm
Why Can’t You Wear White After Labor Day?
Kathy Benjamin
Wearing white in the summer makes sense. Desert peoples have known for thousands of years that white clothing seems to keep you a little bit cooler than other colors. But wearing white only during the summer? While no one is completely sure exactly when or why this fashion rule came into effect, our best guess is that it had to do with snobbery in the late 1800s and early 1900s.
(...)
Not everyone followed this rule. Even some socialites continued to buck the trend, most famously Coco Chanel, who wore white year-round. But even though the rule was originally enforced by only a few hundred women, over the decades it trickled down to everyone else. By the 1950s, women’s magazines made it clear to middle class America: white clothing came out on Memorial Day and went away on Labor Day.
 
Bustle
September 2, 2014
CAN YOU WEAR WHITE AFTER LABOR DAY? A BRIEF HISTORY OF THAT INCREDIBLY OUTDATED FASHION RULE
TYLER ATWOOD
No one appears to have a solid answer to the “no white after labor day” fashion debate. In fact, Labor Day seems to have become a confusing fixture for the fashion community which involves standing in front of your closet and wondering if wearing a white dress will incite the wrath of street style stars and random passerby. A number of feasible theories have been presented over the years. Some blame New York’s turn of the century high society for the peculiar practice; others present scientific claims pinpointing the trend to life before air conditioning; and a select few suggest that the color was a form of societal segregation, keeping the wealthy visually distinguished from those less fortunate. And with arguably over a century of backup, is it really acceptable to finally give this age-old sartorial dictum the proverbial boot?
(...)
By the turn of the century and the advent of the Gilded Age, white had become the lawn-party-in-the-Hamptons, yacht-appropriate, mint-julep-sipping clothing shade of choice for America’s vacation-bound elite.