Ekadashi Shraddha could be performed for those souls who have departed on EkadashiTithi.
Deceased may be died in any Paksha or Lunar Fortnights i.e. Shukla Paksha (Bright Fortnight) or Krishna Paksha (Dark Fortnight).
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Pitri Paksha 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.
Ekadashi Shraddha is also known as Gyaras Shraddha.
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