My grandmother passed on a day I do not know. Which Shradh should I perform for her?

When the exact tithi of a female ancestor’s passing is unknown, Tritiya Shradh is the most appropriate day to honour her within Pitrupaksha, as this is the designated tithi for female-lineage remembrance. Additionally, Sarvapitri Amavasya on October 10, 2026, covers all unknown-tithi ancestors comprehensively.