very best time to perform tharpana or shrardha - auspicious days

tarpana - The Appropriate Time for Performing Shraddh is given in 8th purana  8.2.2 

a) at the commencement of uttarayan or dakshinayan. 

b)at the time when vyatipat yoga takes place. 

c) at the time when vishu yoga occurs. 

d) during both solar and lunar eclipses. 

e) if troubled by the evil influences of a constellation or a planet. 

f) if troubled by nightmares. 

g)at the time when the Sun changes it's position from one zodiac to another.

The various nakshatras like Anuradha,Vishakha and Shwati are considered to be very auspicious for performing

shraddh- especially, if the day happens to be amavasya.Shraddh performed on such a day pacifies the ancestor's soul for eight years. 

Similarly the combination of amavasya with either 'pushya nakshatra' or 'punarvasu nakshatra' is believed to pacify the ancstor's soul for twelve years.

Some other days which are considered to be very auspicious for performing shraddh are-the third day of the bright half of the hindu month Vaishakh, the nineth day of the bright half of the hindu month Kartik, the thirteenth day of the dark half of the hindu month Bhadrapad and amavasya falling in the hindu month of Magh. A man desirous of performing shraddh can do so on any of the above mentioned days. He should take bath in a holy river and offer 'tarpan'(libation) to his ancestors.