What if I do not know the Tithi (lunar death date) of my ancestor?

If the death date in the Gregorian calendar is known, our Vadhyar can calculate the corresponding Hindu Tithi. If neither is known with certainty, Mahalaya Amavasya (Sarva Pitru Amavasya / Aadi Amavasai) covers all ancestors regardless of Tithi — it is considered universally applicable. This is the safest choice for families unsure of the exact date.