Group Requisitions based on previous 3 days Requisitions and automatically create a PO

Summary:

We have a requirement as below:

1. We generate Purchase Orders for a Specific large supplier two times a week say every Tuesday and Thursday.
2. We have many Requisitions (Approved) created through out the week.
3. Every Tuesday a Purchase Order to be created automatically for all the Requisitions from previous week's Friday till that day i.e., Tuesday.
4. Another Purchase Order to be created automatically on Thursday for all the Requisitions from Wednesday till Thursday.

Reviewed the Grouping Options on the Purchase Agreements but didn't find any option to Group the Requisitions based on above requirement and automate PO creation. Kindly advise.