Answer from Kelley, PHR:
A workweek is not the same thing as your hours of operation. Rather, it defines the 168-hour period (i.e., seven 24-hour days) in which you track your non-exempt employees’ time to determine whether they’re owed any overtime premium pay.
The workweek can start and end whenever you would like (e.g., Saturday at midnight or Tuesday at noon), but it should change rarely, if ever. Changing it to avoid paying overtime would violate the Fair Labor Standards Act and could lead to wage complaints.
Your employees should know what the workweek is. We recommend posting it in a place they’ll see, like a wall in the office kitchen or in the employee handbook.