Answer: Casual Friday

Among the many things that defined the 1990s—overuse of teal and dot-com stock bubbles aside—was the rise of Casual Friday. Although the idea of having a day of casual dress in a corporate environment wasn’t exclusive to the 1990s (a professional manufacturing association known as the Hawaiian Fashion Guild promoted a relaxed dress code with their 1966 “Aloha Fridays” campaign), it’s definitely the decade where it roared into prominence.

The rise in popularity wasn’t accidental either, but instead the result of a very carefully calculated marketing campaign. In 1992, Levi Strauss & Co. sent out promotional material to 25,000 human resources departments around the United States as part of a guerrilla marketing campaign to give their Dockers brand a boost. The material, a large brochure, showed a wide variety of casual office wear and encouraged companies to relax the dress code on Friday.

Although, in little pockets here and there, America had experimented with casual dress prior to that moment, it was then, in the aftermath of the marketing campaign, that Casual Friday became an embedded part of workplace culture.