This can be done with advanced payment terms codes. It looks weird, but it will work. You will need to set up a Work Day Calendar (P00071), a Due Date Rule (P00146) and an Advanced Payment Term Code (P00145).
The way this works is to create a work day calendar that only has one work day per week: Saturday. Based on what you wrote, all due dates should be Saturdays (15 days after Friday). You are required to associate the work day calendar with a business unit (the form calls it a Branch/Plant). I would either create a BU just for that purpose, or use my balance sheet BU. Do not use a branch/plant BU. You must create this calendar for each month of each year. You will have to create the calendar for each year going forward.
Then set up the due date rule as follows:
Based on Date: 1 (Invoice Date)
Days to Add: 15
Fixed Days: 0
Months to Add: 0
Calendar: The Business Unit to which you attached the calendar you created above
Work Day Rule: 2 (Include nonworking days in the due date calculation unless the due date is on a nonworking day. If the due date is on a nonworking day, change it to the next working day.
Note: after you create the due date rule, you can test it with the simulator. The simulator is a FORM exit from the Due Date Rules Revision form or a ROW exit from the Work With Due Date Rules form. You probably want to test 45-60 days to verify that the rule calculates the due date correctly for different dates.
Finally build the advanced payment term code as follows:
Discount %: 0
Discount Due Date Rule: leave blank
Net Due Date Rule: enter the rule you created above.
Please let me know if this meets your needs.
Brad