Who is a Pitri
(1) The departed forefathers. 
(2) The human ancestors, generally up to the 7th generation, to whom Pind and water are offered, and whose food is Swadhaa; and then the Lunar Ancestors of the human race. From Brahmaa, the first Pitaa or Pitaamaha came the 10 Rishi Atri, Mareechi, Pulah etc and then the Agnishvaataa, Saumnyaah, Havishmantaah, Ushmapaah, and Aagyapaah came.

As per Hindu philosophy, the human soul is encased successively in three bodies - (1) the external gross material or corporeal or mortal body (Sthool Shareer), (2) the middle one i.e. Astral body (Sookshma Shareer) and (3) the inner most i.e the Causal body (Kaaran Shareer). When the gross physical body is destroyed due to death, the other two coverings, the astral body and the causal body along with the soul escape and enter in another Sthool Shareer (mortal body). The actions and events performed in a person's life time, gets recorded in the causal body in the form of Kaarmik seeds as Praarabdh (balanced statement of deeds of past life experienced in present life), which later becomes the cause of rebirth.

In Manes inhabitant, the astral body is an exact counterpart of the last youthful physical form of a person in his previous earthly sojourn. The astral body is provided with an astral brain which supplies the cosmic energy to the astral heart, gets distributed in the astral nerves and energizes the body cells. All astral persons enjoy the senses of intuitional feelings (Yogananda, 1946). In course of time, the astral body attains death and the soul with the causal body enters to the causal world (Heaven) along with the Kaarmik seeds.

After the death of a person, the body is destroyed. But the mind, the intellect and the Awareness that activated the body during its life time do not die, but exist as energy thought vibrations. This energy is called as “Pret Aatmaa” and can be energized by offering rice and sesame seeds on all New Moon days - Amaavasyaa days - of the month. The “Pret Aatmaa” time scale is of one day is equal to human 30 days. The Pret Aatmaa will have emotional bonding for 365 human days or for their 12 days. Only if their emotional bonding is shredded they get into a energy level called “Pitri”. This is why for one year “Tithi” ritual is done by the family of the diseased person. After 365 days, there is no more emotional connection. After one year the Pret Aatmaa is transformed in to Pitri, and they can accept offering from any human being of the world. After attaining the Pitri state, the energy vibration becomes fit to be identified with a new human body.

To be qualified for a human body of choice, “Punya” or meritorious debit should be available. If not done any merits while living as a human, at Pitri level at least they can achieve merits by doing a Selfless act. The only Selfless act is to bestow “Energy” to Human beings of the earth, who are not related to them. This action bestows merit for Pitri to attain a human birth.

Those who have Energy debits for Materialistic and Spiritual progress are benefited by Pitri’s blessings. Men, women, children can do this Pitri worship or Tarpan. That is done only after their death. By doing this every month on New Moon day removes many materialistic problems in life and help us to progress spiritually. This is of immense belief.

Honoring every month Ancestors on new Noon Moon day is one of the 5 duties of every Human being. This is done in all ancient cultures. If due to physical ailment someone cannot do it for all the 12 months, then they can do on the most important day called Mahaalaya Ammavasyaa, The dark fortnight of Aswaayuj or Aashwin Lunar month (September-October) which is better known as the Mahaalaya Paksh or the fortnight specially sacred for offering oblations to the departed ancestors. The last day of this period, the New Moon day, is considered as the most important day in the year for performing obsequies and rites.

Pitri Lok
According to Hindu concept, the Pitri Lok nourishes the world by controlling food production and proliferation of biodiversity. In a broader sense the Soorya Mandal (Solar System) which controls the above activities in the world, is considered as the Pitri Lok.  Even the status of Pitri Lok is considered to be at a higher plane than the Dev Lok (habitation of gods); and the Pitri Lok needs special comprehension. As per in Manu Smriti, the Indian time coordination is such that one Earthly year is equivalent to 12 days of Pitri Lok and one day of Dev Lok.

The zodiac belt around the Earth is of 360 degrees which is divided into 12 parts (houses), each of 30° degree shared by respective constellations of that house ipso facto. They are named as Raashi (Sign or House) - (1) Mesh, (2) Vrish, (3) Mithun, (4) Kark, (5) Sinh, (6) Kanyaa, (7) Tulaa, (8) Vrishchik, (9) Dhanu, (10) Makar, (11) Kumbh and (12) Meen. As per Indian astrology, Earth is considered to be static and other planets including the Sun move relatively around the Earth. Sun takes 360 days (one Hindu year) to move around the Zodiac belt and stays 30 days (one month) in one house. The astrological activity, its relation with other planets and its impact on the Earth such as, controlling of weather and spreading of radiations vary during the Suns movement from one house to another.

Accordingly Suns stay in one House (Raashi) is considered as one unit of the Sun or a day in Pitri Lok (360 Earthly days = 12 Pitri days); the dark fort night is of their day for alertness and the bright fortnight is their passive state ie night for sleep (Manu Smriti, I/66).

There should not be any confusion in calculating 30 days of Hindu calendar month as it is counted in 3 ways --
(1) from the Full Moon day (Poornimaa) to the next Full Moon day in North India,
(2) from No Moon day (Amaavasyaa) to the next No Moon day in South India and
(3) from Sankraanti to Sankraanti (the expected day of the Sun enters to one zodiac house) from astrological point of view.

Government in Pitri Lok
The Pitri Lok, as per Hindu mythology, is governed by 31 Pitri personnel viz. 12 Aaditya, 11 Rudra and 8 Vasu.

A closer analysis of these Pitri personals, symbolize the placement of the Sun in different zodiac houses, its intensity of radiation, direction and exposure of the Earth; the details are discussed as follows --

12 Aaditya - Aaditya means the Sun God himself; the name has a root from Aditi the mother of Gods. The Sun (Soorya Dev) is the prime Devtaa among the first seven gods presumed by Vaidik Indians. The existence of the Sun in 12 Raashi of the Zodiac belt as they vary in their positions coupled with varied intensity of radiation and the status of weather, the 12 placements of the Sun is considered as the 12 Aaditya. The Gita Press, Gorakhpur, India, has published 12 beautiful icons of the Sun with keynotes on each, reflecting more on the above philosophy.

11 Rudra - Rudra means forceful, furious, penetrating which is comparable with the spreading radiation of the Sun. Rudra is also a Vaidik age deity (Devtaa), subsequently taken as Shiv.

Out of these 12 Aaditya, respective 12 Rudra are expected to come out, but there are only 11 Rudra. In a year, the Sun exhibits relative progress in the Northern region (Uttaraayan) for six months and the next half period exhibit progress in Southern region (Dakshinaayan) Twice in a year, during transition from the North progress to South progress and vice versa a period of 15 days in each event, the solar radiation is either in diminishing or increasing trend in either of the hemispheres. This low radiation period (15 + 15 days) of one month is exempted from Rudra calculation and accordingly, the total number of the Rudra is counted as 11 only.

8 Vasu - The other personnel - eight Pitri personnel are called Vasu. The name itself of the Earth is Vasundharaa (held by Vasu), which has its origin from Vasu.

Interpretation of Pitri With Vasu, Rudra and Aaditya?
Elite have interpreted parents as Vasu - Devtaa in the world, grandparents as Rudra and great grandparents as the Aadityas; a three step coordination of energy flow from the Sun with gene flow from the ancestors. Moreover, unique comparison with the flow of solar energy from sun is unidirectional and as well we cannot return back our inherited genetic traits to our parents justify the solar system as Pitri Lok.

How to Please Pitri?
Some people suggest that by doing the following they can please Pitri in Pitri Paksh, such as

--going to temple and give some items of food and fruits to all priests in the temple
--donate money to the temple
--donate money to some non-profit organizations
--read some portion of Bhagvad Geetaa
--read many names of Lord Vishnu
--read other prayers to your personal God
--gather family members and arrange for a dinner
--best way is to recite Mantra
--best way is to organize Bhajan with family members and friends
--remembering good thing done by them and offer all prayers for their uplift in heaven or wherever they are etc.

But one can please all the the gods by prayer, Poojaa, Havan etc. Feeding poor people is more meritorious than feeding one's own kith and kin. But Pitri cannot be pleased in this way, unless one performs their Shraaddh with Havan, Tarpan etc and feed and honor Vaidik Braahman on their Shraaddh day with Vastra (clothes), Dakshinaa etc as per tradition. This occasion is not to socialize, actual Shraaddh ritual should be performed for them.

Worshiping Pitri
South Indians worship the Pitri on Makar Sankraanti (falls generally on January 14), the day when the Sun returns from the South equatorial line, the Sun God is greeted for the forward movement to-wards our country. The North Indians worship their Pitri in the first half of Aashwin month (coincides with September), called Pitri Paksh (see below) during the autumn season, the period when the Earth exhibits apparently zero inclination towards Sun. After 23rd September the Northern Hemisphere moves away from the Sun and Southern Hemisphere becomes closer; the North Indians bid farewell to the hot Sun and enter into winter with worshiping to Pitri.

This type of greeting the Sun God by a part of the country and farewell by the other part for a single purpose is the greatest coordination of Vaidik age people from view point of national integration.

Pitri Paksh
For their Sraaddh there is a fortnight assigned separately which comes every year. From Poornimaa of Bhaadrapad to the Amaavasyaa of Aashwin - 16 days, are called Pitri Paksh. During these days Pitr or Pitri or Pitar are satisfied, Braahman are fed, Tarpan to Pitar are offered, and donations are given.

Dos and Don'ts During Pitri Paksh
On the day there is any Shraaddh in the house.
--The men should not shave in the morning but can do in the evening.
--No washing of clothes in the morning time.
--This is specially applicable to women - she should not wash her hair and clothes in the morning, just take bath before cooking food for Braahman man or woman.
--No non-vegetarian food

During the Whole Pitri Paksh
(1) One should not have any hair cut, should not buy new clothes, should not wear new clothes, should not use any new thing, should not start any new venture, means no new thing - just continue your simple life routine.