The days and hours a place normally works — and it is not five days everywhere.
The working week is the pattern of days and hours a jurisdiction normally works. Its maximum length is legislated almost everywhere, under labour codes and the ILO conventions on hours of work; which days it falls on frequently is not.
Five days is common and not universal. Six-day weeks remain normal in parts of Asia and the Middle East, several countries are legislating four, and the days themselves vary — a week running Sunday to Thursday is a working week like any other.
Two independent facts are needed to state one: how many days, and which. A system that assumes Monday to Friday has assumed both, and is wrong about at least one of them for a substantial part of the world.
Rest days are the days the working week excludes. The working week is the positive space — which days are worked, and for how long.
Weekend names the gap. Working week names the run, and the two are complements only where a place has a single continuous rest period.
See also rest day · weekend · working day · split weekend · non working day
We hold this as derived from rest days where we hold them; not asserted where we do not. See what we hold.