Can Saptami Shradh be performed for a parent who died on a different tithi?

The primary Shradh for any ancestor should be performed on the tithi matching their death date. However, Saptami Shradh can be performed additionally for any ancestor as a supplementary offering. The main mandatory Shradh for your parent should be on their specific tithi, and you can include them in the Saptami offering as well without conflict.