Amavasya is a very pious festival celebrated in Hinduism. This day comes once every month, on this day people do not get to see Chandradev. The Amavasya that comes in Pitru Paksha is called Sarva Pitru Amavasya. https://www.narayanseva.org/why-is-sarva-pitru-amavasya-important-in-hinduism/