Why is Pitrupaksha the most important time for Pind Daan according to the Vedas?

The Shatapatha Brahmana establishes that the period of the waning moon (Krishna Paksha) is most propitious for ancestral offerings, because the ancestors are believed to be closer to the earthly realm during this period. The sixteen days of Pitrupaksha (falling in Krishna Paksha of Bhadrapad month) are the direct extension of this Vedic principle. The Amavasya (new moon) at the end of Pitrupaksha is considered the most powerful single day for Pind Daan, because the thinning of the veil between the living and ancestral realms reaches its maximum at the dark of the moon.