Pitrutarpana during grahana should be performed during the grahana itself before moksha.
The day is divided into 5 parts of equal duration from sunrise to sunset. They are called Prahna, Purvahna, Madhyahna, Aparahna and Sayahna.
Parvanasraadham should be performed during aparahna, Aamasraadham (giving of rice and raw banana) in purvahna and vriddhisraadha in prahna.
Madhyahna is suitable for ekoddishtasraaddham