First of all Pitra Dosh is not a Curse of Ancestors for sure. It is well defined by the definition as it is a Karmic Debt of the ancestors which a person is having Pitra Dosh in his horoscope is supposed to pay. In simple language Pitra Dosh is formed in the horoscope of a person due to bad karmas or bad deeds done by Ancestors of that person and the native has supposed to pay that karmic debt by suffering the punishment or by the good deeds done by the person having Pitra Dosh in his horoscope.
So Pitra Dosh does not mean that the ancestors are cursing the person having Pitra Dosh in his horoscope but it means that the Ancestors of such person are themselves cursed.Pitra puja is a way to give respect to ancestors.
For the late departed ancestors prita paksh is observed once in a year in Bhadrapad / Ashwin months into the autumn. Its duration is of 15 days from Purnima (full moon) to Amavas (new moon). We also call it Shradh, Sharadh Pakhwada, Sharadh Parv, by which you get blessing from departed ancestors in which we repay to them by this ritual.
Rohit is always of Ancestors that is why Shradh is always happens in this paksh. In Rohit the moon is near to sun, and the sun and moon is on the same zodiac on the Amavasya. The date on which the parent deceased (deaths) have occurred, on that date yagya/havan/agnihotra are the way to satisfy them, and pray to almighty god that our parent are not with us and we pay homage to them. We are living in human species, is because of them because we are their children. If parent would not have been there then how can we would have been. We pray to god that o God you have given birth to our parents in any of species they get the best age, best happiness, best race, best people, best money enrichment, best reputation, best splendor, exquisite pleasure and peace in that species. By any reason if you do not know the date of their death, then the shradh should be observe on the amavasya day.
Praying through Yagya in front of Maha Agni together our Ahutis will become a small particle and it will go to the almighty god in the universe where they are. God passes on our prayers to our ancestors, where they get, happiness and peace. Ancestors soul will be satisfied like our soul.The reason is, the resolution of glory, because of Yagya and havan we donate the havan element to ancestors. God knows our mental resolution.Vedas, Upanishads, Puranas also approve this. During Shradh, reverence resolution is the only reverence worship. During the Yagya for ancestors by the Bhutyagya Balivaishavdev yagya the following 10 offferings should be given by mixing ghee and kheer.
Om Agney Swaha
Om Somay Swaha
Om Agnishomabhyam Swaha
Om Vishvebhyo Devebhya Swaha
Om Dhanvantarye Swaha
Om Kudhwe Swaha
Om Anumataye swaha
Om Prajapataye swaha
Om Dvaprithivibhayam Swaha
Om Swishtikite Swaha
After that in Peepal/Banana leaf put kheer/pakode/puri etc in the East of Yagya kund, south, West, North, Door, side part, water etc. and give the Balibhag. After that divide the Balibhag in 6 parts and keep it for Dog, Patit, Chandal, Paaprogi, crow and small animals.