Hi @ktpoung,
Try something like the following,
IF({Send Date},IF(WEEKDAY({Send Date})=6,DATEADD({Send Date},-7,'day'),IF(WEEKDAY({Send Date})=0,DATEADD({Send Date},-8,'day'),DATEADD({Send Date},-6,'day'))))
I was testing this really quickly on my end & both Saturday & Sunday dates in a 'Send Date' field would result in a Due date on the friday that is 8/9 days prior - I figured you had a 7 day window here, so setting the due date 1 day, or 2 days prior to the send date would not work for what you are trying to do.
Let me know if this works 🙂