The 16-day long period of Pitru Paksha is meant for seeking forgiveness from the ancestors and offering the Pind Daan (food) to satisfy their hunger. It is a time to perform penance, correct themselves, ask for mercy, maintain celibacy and lead a humble life. To understand why we must not indulge in worldly things during this period, let us learn about the legend associated with Karna.
Karna, the eldest son of Kunti, was referred to as Daan Veer because of his philanthropic deeds. He gave everything that people asked for. The eldest Pandava donated even his prized possessions, including the Kavach and the Kundal that shielded him from death.
However, after his death, when he went to the Pitru Loka, he found everything that could comfort him but not the essential need, i.e. food. He wondered why he had to remain hungry for days despite having been so magnanimous during his lifetime. Therefore, to answer his question, Yamraj, the God of death appeared and explained why he went wrong. Hence, Yamraj gave him a new lease of life for a fortnight so that he could return to earth and donate food to people. Karna had also not performed the Shradh and Tarpan for his ancestors. Hence, he returned to the world, donated food and even did the Shradh to please his dead ancestors.