Pitr Paksha~Shradh~श्राद्ध पर्व - Story about Pitru Paksha
The story why the Pitru Paksha is marked by daan and charity has its deep ingrained meaning in a story of the Mahabharata.
When Karna died in the epic Mahabharata war, his soul moved to heaven, where he was offered gold and jewels as food. Karna had done limitless charity of wealth, but had neglected to do Anna-Dana. Hence he found wealth and luxuries, but with no food to appease him.
Lord Yama then told Karna that he had donated gold all his life, but had never donated food to his ancestors in Shradh. Hence to make amends, Karna was permitted to return to earth for a fifteen day period, so that he could perform Shradh and donate food and water in their memory. This period is now known as Pitru Paksha.
It is believed that in these 15 days the souls of our ancestors are the closest to the earth, moving around us and as we fail to recognize them, they return to their place unsatisfied and unhappy that we do nothing for them.
The fifteen days of Mahalaya Paksha consists of 15 Tithi. They are Pratipada, Dvitiya, Tritiya, Chaturthi, Panchami, Shashti, Ashtami, Navami, Dasami, Ekadasi, Dvadasi, Trayodasi, Chaturdashi, New Moon day (Sarvapitru Amavasya).
According to Hindu mythology, every individual’s who wants to perform this Pitru Tharpanam, they should do it on the same day of their ancestor died which will fall within any one of these fifteen days.