We don't know the date of death — when do we do śrāddha?

on Sarvapitṛ Amāvasyā, the last day of Pitṛ Pakṣa, which is meant for all ancestors.

It is very common not to know the tithi on which a grandparent or great-grandparent died, or to have only the English calendar date. For exactly this reason tradition provides Sarvapitṛ Amāvasyā (also called Mahālaya Amāvasyā), the new-moon day that ends Pitṛ Pakṣa. A śrāddha or tarpaṇa done on this day reaches all ancestors — known and unknown, remembered and forgotten.

There are also a few special days within the fortnight that many families use. Mātṛ Navamī, the ninth day, is often kept for mothers and other women of the family who passed away. In many traditions the caturdaśī (14th day) is kept for those who died suddenly, by accident or violence.

If you know the English date of death, a pañcāṅga or astrologer can work out the tithi for you. But do not let uncertainty stop you; Sarvapitṛ Amāvasyā is always correct.

Source: Nirṇayasindhu; common practice · Mātṛ Navamī: for mothers

Dharma Q&A — family & life events · Ask a Jyotishi your own question