A workweek is defined by and varies by employer. It is not required to be scheduled as a calendar week from Monday through Sunday. A work week, for some people, can be Monday through Friday, or it can be Monday through Saturday--and depending on the job. The workweek ordinarily includes all time during which an employee is necessarily required to be on the employer's premises, on duty or at a prescribed work place.
A workday, in general, means the period between the time on any particular day when such employee commences his/her "principal activity" and the time on that day at which he/she ceases such principal activity or activities.
The US Department of Labor does not define the work week but gives restrictions on the requirement of overtime pay after 40 hours worked.