A fashion week is a fashion industry event, lasting approximately one week, wherein fashion designers, brands or “houses” display their latest collections in runway fashion shows to buyers and the media. These events influence trends for the current and upcoming seasons.

The most prominent fashion weeks are held in fashion capitals of the world, with the “Big Four” receiving the majority of press coverage being New York, London, Milan, and Paris.
While the fashion scene turns more multipolar in the 21st century, other centers like Mumbai, Beirut, Berlin, Dubai, Los Angeles, Madrid, Monaco, Rome, São Paulo, Taipei, Shanghai, New Delhi, Copenhagen, Sibiu, Jakarta, and Tokyo host important fashion weeks.
Paris fashion parades
The concept of fashion week (Semaine des défilés (fr)) began in Paris, when marketers would hire women to wear couture items in public places, from racetracks to salons.
These parades (parade is “défilé in French) gradually began to become social events of their own. (Indeed, in French, runways shows are still called “défilés de mode” — literally “fashion parades” — today.)
The first Paris fashion week began in 1973.


New York and the first “fashion week”
In 1903, a New York City shop called Ehrich Brothers put on what is thought to have been the country’s first fashion show to lure middle-class women into the store.By 1910, many big department stores were holding shows of their own. It is likely that American retailers saw what were called “fashion parades” in couture salons, and decided to use the idea. These “parades” were an effective way to promote stores, and improved their status. By the 1920s, the fashion show had been used by retailers up and down the country. They were staged, and often held in the shop’s restaurant during lunch or teatime. These shows were usually more theatrical than those of today, heavily based upon a single theme, and accompanied with a narrative commentary. The shows were hugely popular, enticing crowds in their thousands – crowds so large, that stores in New York in the fifties had to obtain a license to have live models.

https://www.vogue.com/fashion-shows