Pitri Paksh is not a festival as such but it is observed every year in most part of India as a worship Paksh (fortnight) to one's Pitar or ancestors. This fortnight falls in first fortnight or Krishn Paksh of Aashwin month of Indian lunar calendar. In fact it starts from Bhaadrapad Poornimaa and lasts for 16 days, till Aashwin Amaavasyaa. Its purpose of being of 16t day is that its each day is devoted to the day a man can die on. For example if a person dies on Ashtamee (8th day) his Shraadh will be observed on the Ashtamee in this Pitri Paksh. It does not matter whether he died in Krishn Paksh or Shukla Paksh, it is only Tithi is seen in this regard. Since there are 16 Tithi every month - 14 Tithi, one Amaavasyaa and one Poornimaa; these 16 days are assigned to all those 16 Tithi in this Pitri Paksh. This Paksh is called "Shraaddh Paksh" also, because in this Paksh Shraaddh for one's Pitar (ancestors) are performed.

This is a very special Paksh in whih one has live piously to please his ancestors. Mostly people follow these restrictions in this Paksh -
--They do not get their hair cut - many people do not even shave the whole Paksh. But it is necessary not to have hair cut and shave at least on the day the Shraaddh is performed.
--They do not give their clothes to laundry, they can wash their clothes at home. But the day there is any Shraaddh in the house even washing clothes at home also is prohibited.
--They do not wear any new cloths, do not buy any new thing, and do not use any new thing.
--They do not eat non-vegetarian food and do not drink liquor etc for the whole Paksh; and do not eat even onion and garlic etc on the day when they have to perform any Shraaddh.
--Both the Braahman and the performer do not travel on the day they perform Shraaddh.
--Shraaddh should be observed on the respective month, respective Paksh and respective Tithi of father's and / or mother's  death day, as the case may be. If the concerned Tithi falls on two days, it must be observed on the day in which the Tithi lasts after 1-30 PM and on which day the duration of the Tithi is longer. If the duration of Tithi is equal on both days, then it should be done on the first day. If the Tithi concerned falls twice in a month then it should be performed on the later one. If Sankraman Dosh occurs on the second one, then the earlier one is the right one for performing Shraadh. If Dosh occurs on both the Tithi, Shraadh should be observed on the later Tithi. If there is no Tithi in a month it can be performed in the previous month subject to verifying Chaandramaan Shuddh Tithi.
--If the Shraaddh performer becomes polluted on account of A-Shauch (arising out of death of blood relatives), Shraaddh should be performed on the day he is freed from A-Shauch.
--Women do not wash their heads on the days a Shraaddh is performed in the house.

Shraaddh should be performed at Kutap Kaal. If the day (after sunrise) is divided into five parts, first part is called Prataah Kaal followed by Sangav Kaal, Madhyaahn Kaal, Aparaahn Kaal and Saanya Kaal. If the Madhyaahn Kaal is divided in three parts, the first one is called Gandharv Kaal, then Kutap Kaal and Rauhin Kaal.

In a place where the Sun rises at 6 AM, and sets at 6 PM, Kutap Kaal falls in between 11-36 AM and lasts till 12-24 PM. This time will vary from place to place depending upon time of sunrise and sunset. Hence the right time to perform Shraaddh is around 12 noon.

--Apart from Kutap Kaal, black sesame seeds and Darbhaa grass are significant in Shraaddh. For any reason, if one is not able to perform Shraadh on relevant Tithi, he should abstain from eating that day and perform it on the following day.

--One not knowing Ved and one who is sick should not be selected to officiate as Shraadh Braahman.
--Brothers can not be selected as Shraadh Braahman.
--Bachelors and one who is married but not having issues is not eligible to officiate as Shraadha Braahman.

--If for any reason, if one is not able to perform Shraaddh on relevant Tithi (or one has forgotten the Tithi), he may perform it on the last day of the Pitri Paksh - Amaavasyaa. All forgotten Shraaddh may be performed on this Amaavasyaa, that is why it is called Pitar Samokinee Amaavasyaa. Many people perform all of their Shraaddh on this day.

The day a Shraaddh is performed in the house, the house is cleaned well and good food which was liked by the person whose Shraaddh is being performed on that day is cooked. One, two or four Braahman, according to one's need an Shraddhaa, are invited for food on the same day at early morning. Those Braahman perform Tarpan ceremonies, are fed in the afternoon, are given Dakshinaa or any other thing (such as clothes or jewelry) and are bade farewell with Shraddhaa and Bhakti.

All these restrictions end at the end of the Paksh on Aashwin Shukla Pratipadaa (1st day of Shukla Paksh) or on the first day of Nava Ratri day.

Significance of Donating Food During Pitri Paksh
The easiest way to access God's grace is to feed a famished human being starving for food. Donating food is considered to be the greatest charity of all the charities and it is believed that whoever donates food will attain an 'elevated status'. There is an interesting legend highlighting the importance of donating food from the great epic Mahaabhaarat.

The renowned hero of the Mahaabhaarat, Karn, was a great donor and he never said no to anyone who approached him for help. He had donated huge wealth and had done all kinds of charity, except for donating food. When Karn left his earthly body after his death and reached Heaven, the great charity which he had done on the earth was returned to him hundredfold here, but it was all material wealth and no food at all. He then realized that although he had done all kinds of charities, he did not donate food. He prayed to the God of Death that he should be sent oln Earth for a few days so that he could donate food. Yam Raaj sent him back to Earth for 14 days, to make up for this deficiency.

Karn came here again, fed Braahman and poor and offered oblations of water during these 14 days, which are observed in the Pitri Paksh. On his return to Heaven, he had plenty of food and it has been anticipated that offerings made during the period of Pitri Paksh benefits all the departed souls, whether they are connected to you or not.