Paris Fashion Week closes the Big Four each season and is widely regarded as its pinnacle — the home of luxury, heritage houses, and haute couture. It is organised by the Fédération de la Haute Couture et de la Mode.
When it runs
Paris shows last each season, closing the month of shows — late February to early March for autumn/winter, and late September to early October for spring/summer. Haute couture is shown separately, typically in January and July.
What defines it
Paris is the historic centre of fashion and the summit of the calendar: the home of the great luxury houses and of haute couture — made-to-measure, hand-finished clothing that is a legally protected designation in France. It is organised by the Fédération de la Haute Couture et de la Mode, which oversees the official Paris schedule.
Sources: Paris Fashion Week — Wikipedia · Facts summarised in our own words; text under CC BY-SA where sourced from Wikipedia.