As per our religious literature after death of a person, the body is destroyed. But the pattern of the thinking mind and the intellect that is imprinted in the Awareness does not die, This Energy-Awareness without a manifesting body is called as "Praetha-Atma” and is energized by our “Energy thoughts” through the ritual of offering rice and sesame seeds on all new moon days of the month. The "Praetha-Atma" is in a different energy level and so the time scale is different from the human time scale. 30 days of human time is equal to one day of Pretha-Atma. So every new moon is the morning of the Pretha-Atma. Like we break our night fast and energize the body with food, the Preta-Atma also need to be Energized to get into the next human body. The Pretha-Atma will have emotional bonding for human time scale of 365 days (12 months) or their 12 days. Only if their emotional bonding is shredded they get into a energy level called "Pitru". This is why for one year "Thithi" ritual is done by the family of the diseased person. After 365 days, there is no more emotional connection to the family. After one year of transforming to Pitru, they will accept offering from any human in the world. After attaining the Pitru state, the energy vibration becomes fit to be identified with a new human body.one can do rituals for the pitra according to the panth (as told by particular sect, astrologer or brahmin )
The main aim is to give tribute and seek blessings of the ancestors.Rishi Bhrigu did a lot of penance for the ancestors.
Honoring every month Ancestors on new moon day is one of the 5 duties of every Human being. This is done in all ancient cultures around the world in different ritualistic mode. If due to physical ailment some one cannot do the 12 months, then they can do on the most important day called Ammavasi,
Take Bath and visit temples. Give alms to poor, food to needed.