Why is Pitrupaksha observed only during the Krishna Paksha and not any other fortnight?

The Krishna Paksha (waning/dark fortnight) is traditionally associated with ancestral energy, the realm of the dead, and the moon’s influence on the subtle planes. The Chandogya Upanishad describes the moon as the gateway through which souls travel between realms — making its phases directly relevant to ancestral rites. The waning fortnight, when lunar energy withdraws, is considered the time when the gates to Pitru Loka are most open, making communication and offering most effective.