Hi,
What I would do for this requirement is the following:
1) Add a new custom field ZINVDATE type DATS in table EITR (you can do this in CI_EITR)
2) Create a new Z program that will go through all records in EITR and decide based on your own rules, how many invoice should be created on that date and change the field ZINVDATE to today's date. This program shall be executed before EA26.
3) in EA26 you should see the new ZINVDATE field in the custom selection. You need to select that field and enter today's date. EA26 will then just select the invoices you have selected to print today and leave the other ones in EITR tables for next days.
Hope this helps!
Daniel