In the early parts of the 20th century, fashion etiquette begun dictating that wearing white was only acceptable in summer months, according to Time Magazine. Labor Day was chosen as the endpoint to the season.
Since white and other light colors are better at reflecting sun and keeping the wearer cool, light colors and breathable fabrics are obviously popular in hot climates and during warmer seasons. Time Magazine states that the summer months of the early 20th century proved important to socialites and other wealthy Americans. The summer was a time to leave city responsibilities behind for long vacations at summer resorts, and white outfits became associated with summertime leisure. Memorial Day in the end of May and Labor Day in the beginning of September were the two long weekends that became natural start and end dates for the summer season, and so it followed that people used these dates to know when to pull the white outfits out of the closet.
Like many fashion rules, knowing when to wear white was also an unspoken way of determining social acceptability. In a time of growing class mobility and "new money," certain elements of upper-class social codes became more rigid and fiercely observed in an attempt to preserve elite society. By the 1950s, the popularity of women's magazines had turned this upper-class distinction into a broad piece of fashion advice for all women and men, as explained by Mental Floss.
The fashion rule about wearing white after Labor Day remains in place in more conservative style guides even today; however, many etiquette guides, fashion magazines and advice columnists consider it an outdated rule.