In Hinduism, Pitru Paksha is the most important time for worshiping forefathers.
The Krishna paksha of the solar month Kanya is called Pitru paksha. During this time, the Sun passes through the stars: Uttara phalguni, Hasta, and Chitra. Pitru Paksha includes the subsequent Shukla Paksha Pratipada also.
As soon as the sun enters Kanya rashi, all the forefathers come out of Pitruloka. They go to their descendants. They are hungry and thirsty and hope to receive food and water from their descendants. If they are disappointed they curse and go back.
Every comfort is provided to the occupants of Swargaloka. Forefathers who are in Pitruloka have to get all they need from their descendants on earth. That is why shraddha is performed. Forefathers remain hungry and thirsty if shraddha is not performed at the specified time.
Yamaraja does not allow the forefathers to stay in Pitruloka during Pitru Paksha. They have to come down to earth to visit their descendants. It is the descendents' duty to welcome them and to provide them with food and water. In the hope of being invited inside, the forefathers wait at the doorsteps of their descendents’ homes.
In the olden days, during Pitru Paksha shraadha used to be performed every day following the complete procedure. Nowadays, most people perform tarpana using tila every day during Pitru Paksha and shraddha on any one day during the Paksha. In case you are not able to observe the entire Paksha, you can start from Panchami, Ashtami, or Dashami and perform till Shukla Paksha Pratipada.
Dwadashi of Pitru Paksha is meant for sanyasis who have attained samadhi.
The Trayodashi of Pitru Paksha is called Gajachhaya and is very auspicious for performing shraddha. The Moon should be in Magha Nakshatra as well.
Bharani nakshatra day during Pitru Paksha is very important. Doing shraddha on this day is as good as doing Gaya shraddha.
Observing Pitru Paksha is no less than performing a yajna. It is not even optional. It is mandatory for every religious Hindu
Glory of Venkatesha
Story of Sri Yantra