You must have the authority permission Scheduling: Planning to change the day of the week on the Planning Board, or the Staffing Board. To change the default day of the week at the Institution level, you must have the authority permission, Setup: Organizations.
To change the start day of the week:
Planning Board (User only): Go to the Planning Board > click the gear icon > select Display > choose a new day.
Staffing Board (User only): Go to the Staffing Board > Blueprints > Select Blueprints > Planning Board opens > click the gear icon. Select Display Settings > choose a new day.
Institution (Global setting for the selected institution): Go to the Setup > Organizations > select Institution > click Edit > in First day of week default, choose a new day.
Changes apply to the Planning Board, and the Staffing Board.