Pitru Paksha is a 16 days period when one offers reverence to their ancestors in the form of Tarpan or Shraddh Puja. As per Brahma Purana and other Vedic scriptures, worship and rituals offered during the time of Pitru Paksha helps souls of one’s ancestor attain peace and move on to the next realm.
As per the ancient Indian history, when Karn passed away during the war of Mahabharat and his soul reached heaven, he didn’t get to eat regular food. Instead, he was given gold and jewels to eat. His soul got frustrated and he addressed this issue to Indra (Lord of heaven) that why is he not being served the actual food? Then Lord Indra revealed the actual reason that He donated all these things to others throughout his life but never gave the same to his ancestors. Then Karna replied that he didn’t know of his ancestors and after hearing him out, Lord Indra allowed him to return back to Earth for the period of 15 days so that he can donate food to his ancestors. This 15-day period came to be known as Pitru Paksha.