Pitru Paksha is a 15 lunar days period when Hindus pay homage to their ancestors, especially through food offerings.
According to South Indian Amavasyant calendar it falls in the lunar month of Bhadrapada beginning with the full moon day or day after full moon day.
According to North Indian Purnimant calendar this period falls in the lunar month of Ashwin beginning with the full moon day in Bhadrapada or next day of full moon day.
It is just nomenclature of lunar months which differs and both North Indians and South Indians perform Shraddha ritual on similar days.
The last day of Pitru Paksha is known as Sarvapitri Amavasya or Mahalaya Amavasya. Mahalaya amavasya is the most significant day of Pitru Paksha.