Dwadashi Shraddha could be performed for those souls who have departed on Dwadashi Tithi.
Deceased may be died in any Paksha or Lunar Fortnights i.e. Shukla Paksha (Bright Fortnight) or Krishna Paksha (Dark Fortnight).
Dwadashi Shraddha Tithi is most appropriate for performing Shraddha for those who had taken renunciation (संन्यास) before death.
Read More : Cost of Pind Daan/ Asthi Visarjan/ Nagbali/ Narayan Bali/ Tripandi etc.
Dwadashi Shraddha is also known as Baras Shraddha .
Pitri Paksha or Mahalaya Shraddhas are Parvan Shraddhas(पार्वणश्राद्ध) and auspicious time to perform them is Kutup Muhurt and Rohina Muhurt.
It could be performed till Aparahna Kala (Post Meridian) gets over.
Tarpan (तर्पण) could be performed at the end of the Shraddha.
Read More : Pind Daan Packages