How do you create a calculated field that will show same day count/sales from last week (-7 days) in a report i.e. Monday sales this week and Monday sales last week?

I have been trying to find a calculation that will allow me to show the sales data for a week day and then the sales data for the same day last week. I have the date calculation part figured out pretty easily, but I can't find the solution to getting the sales numbers to populate.

I am not looking for a summary but just a straight number from the prior week day.

I am guessing this will be a simple solution but I just cannot find it.