According to the Hinduism, it is believed that the soul of the passed away ancestors of a family reside at the Pitra Loka i.e a realm between the heaven and the earth visit earth at the time of Shraddh Paksha or Pitra Paksha. The soul of the ancestors visit their home with a desire of pind-daan or tilanjali and if someone doesn’t perform it, they go back unhappy. The ceremonies and Tarpan performed at this time for their peace is known as Shraddh. Thus, it is believed that whatever we donate in the name of our ancestors is received by them also.
For the welfare of the family we have to do Shraddh for our ancestors with all the devotion and love to take their blessings, without them we cannot exist and without their blessings we might face many difficulties of life, so it is must, Shraddh is to be with all piousness and purity,
Father shraddh is to be done by son, if son is not alive or not born then wife can do the shraddh for her husband, if wife is not alive, then real blood brother need to do shraddh, If there are more then one son, then eldest son need to do the shraddh for their father, the husband of daughter or daughter kid can also do the shraddh, if son is not alive then grandon or great grandson need to do the shraddh,
If son, grandson or great grandon is not alive, then the widow of deceased husband can do shradh karma, Wife shraddh can only be done if son there is no son available, if son, great grandson or daughter son is not available then brother son can also do the shradh karma, even the adopted son is righteous to do shradh,
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