The last day of Pitru Paksha is known as Sarva Pitru Amavasya. This Amavasya is considered especially significant for remembering all known and unknown ancestors whose Shradh date may not be known or whose Shradh could not be performed on the prescribed tithi for some reason.
Indira Ekadashi, which falls during the Krishna Paksha of the Ashwin month, is one such meritorious tithi. As it occurs during Pitru Paksha, this day is specially associated with the worship of Lord Vishnu along with feelings of reverence and righteous deeds.
Our ancestors laid the foundation of the family through their lives, austerities, values, and good deeds. Pitru Paksha is the sacred opportunity to express reverence and gratitude towards them.