If your system is automatically set for the 'start day' for the week to be one day (Sunday, Monday), but you need to use another day, you can follow these steps to change the day.

1. I have a customers table, and I want to get a count of customer sign ups per week starting on a Sunday instead of Monday. First I need to click on the date column I'm using, and add this formula: 
   my_date_column.toDateTime() + 1.day()
2. Next, do a row count broken down by the date column you're using, set to week.
3. Then select the column header drop down from the date column you have, select new column, and enter this formula:
   my_date_column.toDateTime() - 1.day()