Sarva Pitru Shraddha is and can be performed by all for the ancestors who passed on Amavasya, Poornima, on both sides of the family and for those ancestors whose Tithi (Indian calender dates) you do not remember.
So say for example you remember one or two people’s dates for shraddha but you do not know or do not remember your great grand parents dates of passing, then the last day of shraddha which is know as sarva pittru shraddha is to be used to cook and feed the pancha maha bhoot so all the ancestors can come together just like a big party for the ancestors and enjoy the day at your place. Give you/us blessings for Health,wealth,happiness,peace, progeny and prosperity etc. Remember to cook with lot of love whatever you can and offer.
Until next time. Please shre and pass it on to he next generation.