Mahalaya Amavasya is observed on the no moon day (Amavas) in the month of Ashvin or Ashvayuja (September-October). Mahalaya Amavasya 2017 date is on September 19. Mahalaya should be performed in the morning. As it is falling on a Monday the day is highly auspicious and is known as Somavati Amavasya. It is the final and the most important day during the Pitru Paksha Shradh fortnight. The popular belief is that performing Mahalaya Shradh is equal to performing Shradh and Tarpan on all days of Pitru Paksha.
Mahalaya Amavasya also marks the beginning of Durga Puja rituals. Goddess Durga descends on to earth on the day.
It is said that those people who failed to observe Shradh rituals on a particular day during Pitru Paksha fortnight can solve it by observing Mahalaya Amavasya Shradh.
Another popular belief suggests that the souls of those people who had unnatural deaths like accident etc are satisfied and rest in peace when their relatives or sons or daughters perform Mahalaya Amavasya Tarpan rituals.