Yateenam Mahalaya or Yati Dwadashi Shraddh is observed on Bhadrapada bahula Dwadashi, the twelfth day during Mahalaya Shraddh Paksha. In 2020, Yateenam Shraddh date is September 14.
Yatis, also spelt as Yetis, are the legendary people who prepared scriptures, Puranas, Upanishads and provided the knowledge package for us. Yati is also a synonym for Rishi or Muni.
On Yati Dwadashi Shraddh, Hindus perform santarpana to Brahmins and some simple rituals related to shraddh. It is observed in South India only, mainly in Karnataka and Andhra Pradesh.