Pitru Paksha (पितृ पक्ष) is a 16–lunar day period in Hindu calendar, when Hindus pay homage to their ancestors.
Pitru Paksha is considered by Hindus to be inauspicious, given the death rite performed during the ceremony, known as Shraddha(श्राद्ध) or tarpan(तर्पण). It begins on the full moon day of Bhadrapada(भाद्रपद पूर्णिमा) ending with the Sarvapitri amavasya(सर्वपितृ अमावस्या,आश्विन कृष्णपक्ष अमावस्या)
According to Mahabharata, when Karna died, his soul transcended to heaven, where he was offered gold and jewels as food. However, Karna needed real food to eat and asked Indra, the lord of heaven, the reason for serving gold as food.
Indra told Karna that “he had donated gold all his life, but had never donated food to his ancestors in Shraddha (श्राद्ध). Karna said that since he was unaware of his ancestors, he never donated anything in their memory”
To make amends, Karna was permitted to return to earth for a 15–day period, so that he could perform Shraddha and donate food and water in their memory. This period is now known as Pitru Paksha (पितृ पक्ष)