What is Pitṛ Pakṣa?
a fortnight each year, usually in September–October, set aside to honour and feed our ancestors.
Pitṛ Pakṣa (also called Śrāddha Pakṣa or Mahālaya) is the dark fortnight of the lunar month Bhādrapada in the southern (amānta) reckoning, called Āśvin in the northern (pūrṇimānta) reckoning. It begins after the Bhādrapada full moon and ends on Sarvapitṛ Amāvasyā, the new moon. Tradition says the ancestors are especially close to the earthly world in these days and receive what their descendants offer.
Each ancestor's śrāddha is done on the tithi (lunar day) on which they died, whatever the month of death. So if your grandfather passed away on a ṣaṣṭhī, his śrāddha falls on the ṣaṣṭhī of Pitṛ Pakṣa. Families offer tarpaṇa (water with sesame), feed priests or the needy, and set aside food for crows, cows and dogs.
Many families avoid new purchases, weddings and housewarmings in this fortnight. It is a time of gratitude, not fear.
Source: Nirṇayasindhu; Garuḍa Purāṇa · Ends on Sarvapitṛ Amāvasyā
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