Shraddha is the Hindu rite of offering food, water and prayers to departed ancestors (Pitru).
Pitru Paksha — the 16-day lunar period in Bhadrapada when the Sun is in Virgo. Ancestors are believed to descend to Earth during this period.
Mahalaya Amavasya — the last day, also called Sarvapitri Amavasya. Suitable for all ancestors regardless of death Tithi.
Shraddha is performed on the same Tithi the ancestor passed away during Pitru Paksha.