During the era of Varaha Avatar, after restoring the Earth from the deep ocean waters and killing Hiranyaksha, Sree Maha Vishnu in the form of ADI YAGNA VARAHA, was drenched in muck.
It was mid-noon. He plucked a few of his hairs, that became DARBHA, and from his skin pores, brought forth Black sesame seeds or TILA. He put the DARBHA on the ground, and from the mud of his body, he made 3 balls, placed them on the DARBHA, and with the TILA he first did PINDA PRADHANA to the Pitru devatas.
When the Devatas asked about the same, he said
I am the Pitrudevatas and I am also the father, grandfather and great grandfather of the entire Cosmos. From now on, everyone must perform PITRU KARMA without fail.
This ritual of Pitrukarma was taught to the universe by Sree MahaVishnu himself.
Pitrudevatas are the progeny of the Devatas and are responsible for the travel of Jeevatma to different realms according to their Karma.
Everyone, including Devas, should perform Pitrukarma with same austerity and shraddha as they do Devakarma or Yagna.
When Jeevatma (soul) attains a body (birth), it is put to many troubles and turbulences according to its Sanchita karma. Pitrudevatas are spread across the entire Cosmos in different categories and are protection to the Jeevatma in this Janma and other realms after leaving the body.
There are 31 PITRU GANAS according to NARAYANEEYAM, who are divided into 7 categories according to the benefit they bestow.
PITRU DEVATAS are of two types:
MOORTHA (Those who have a distinct form) PITRU DEVATAS
POORA
BHAVASARA
VASU
SHUKALA
AMOORTHA (who are formless) PITRU DEVATAS
VAIRAJA
AGNISHWARTA
BARHISHADA
All the 31 PITRU GANAS are from these 7 categories, they are an invisible sheath of protection to our lives. Pitrukarmas energise these devatas, which in turn become an invincible protection from negative and evil forces that create unnecessary material and otherwise obstacles, which in the form of failure, pain, suffering, unhappiness and loss of progeny, try and obstruct the elevation of the Aatma.
The Drik Pitrudevatas which protect us from four directions are:
AGNISHWARTA in the East
BARHISHADA in the South
AADHYAPA in the West
SOMUKHA in the North
Apart from these there are 31 PITRU GANAS, which bestow us and protect in the material realm.
First category consists of 9 GANAS:
VISHWA, VISHWABHUK AARADHYA, DHARMA, DHAANYA, SHUBHAANANA, BHUTIDA, BHUTIKRUT, BHUTI SWARUPA.
As their names suggest these 9 GANAS help us in determining or etching our direction in this janma.
The second category consists of 6 GANAS:
KALYADA, KALYAKARTA, KALYATARASRYA, KALYANA, KALYATHAHETHU, ANAGHA
These 6 GANAS help in fructifying the direction that we etch for ourselves in lives.
The third category consists of 7 GANAS:
VARA, VARENYA, VARADA, THUSHTIDA, PUSHTIDA, VISHWAPATHA, DATHA
These 7 GANAS make our efforts successful .
The fourth category consists of 5 GANAS:
MAHAAN, MAHATMA, MAHITHA, MAGIMANAYA, MAHABALA
These 5 GANAS help our success become noticeable and increase our fame.
The fifth category consists of 4 GANAS:
SUKHADA, DHANADA, DHRAMADA BHOOTADA
These 4 GANAS help consolidate happiness in the form of progeny.
Such astute science is embedded in the PITRU DEVATA VIGNANA.
To lead a life of fulfillment and Happiness in all avenues, PITRU DEVATA KRUPA is extremely essential.
According to NARAYANEEYAM, every person who obtained a physical form of human has to perform 96 Shraddha Karmas annually. As this is not possible for various reasons in Kaliyuga, one should at least perform Pitru Karmas for the following occasions in a year:
Every Amavasya
Mahalaya Paksha,
Annual Shraaddha Tithi of Departed Ancestors.
If, for various reasons, one isn't able to perform Pitru Tarpana on every Amavasa, they must feed Gowmatas and do Pitru Tarpana in Mahalaya Paksha or at the least get it done, for their own good.
GOSAVITRI STOTRAM from Anushasika Parvam of Mahabharata, says
खुराणां पश्चिमे भागे ह्यप्सराणां गणाः स्मृताः॥१०॥
श्रोणीतस्तेषु पितरो रोमलांगूलमाश्रिताः।
Meaning: the deities representing the pitru devatas, are found in the hairs of the bodies of Gowmata and also on the hairs at the tip of her tail.
Upon offering fodder to Gowmata, in these 15 days of Mahalaya Paksha,
she heaves a sigh of relief, that helps the Pitrus reach a higher realm [OORDHVA LOKA].
When Karna died in Kurukshetra war, his soul transcended to Heaven. He became plagued with extreme hunger, as whatever he touched transformed to gold. Indra told Karna that though he donated gold all through his life, he had never fed a Gowmata in his life for his ancestors on Amavasya or Mahalaya Paksha, and hence was cursed by his pitrus, which explained the reason for his unstable life when alive and trouble after death. Upon request to make amends, Karna was sent back to earth for 15 days in MAHALAYA PAKSHA to perform Shraddha Karmas for his Pitrus by feeding Gowmatas.
Swayam SREE MAHAVISHNU says that when Gowmatas are fed, all the Pitrus spread in different realms in the cosmos would be satisfied and they would bless and protect us in all avenues.
Amavasya Tarpana is done amidst the Gowmatas, for your ancestors upon registering the details.