In utilization calculations the Week Start Day field is used to determine what This Week and Last Week are. For instance, when scheduling utilization calculations if you set the Historical Utilization Cut-Off Date on the Utilization Calculator page to End of Last Week and the Week Start Day is Sunday, each time the calculation runs the cut-off date is the day before that.
In backlog calculations the Week Start Day field determines how the Start Calculating From field works. For instance, if you set the Start Calculating From field on the Calculate Backlog page to Start of This Week and the Week Start Day is Sunday, the Start Date is set to the most recent Sunday. This is used to determine the relative start date when scheduled backlog calculations run.
When generating missing timecards this setting is used to determine the week start day.