Tharpanam is an offering to Gods or Pithru devatas (dead ancestors). Tharpanam offered to Gods is named on the basis of God to whom we offer Tharpan – viz. Brahma Tharpanam, Surya Tharpanam, etc. The offering which we make to our dead ancestors is Pithru Tharpanam. Generally, Pithru Tharpanam is done during Amavasya days, Solar and lunar eclipse days, Sankraman or Sankranti days, during Shraddh, and in Shraddh Paksha or Pitru paksha or Mahalaya Paksha.
It is believed that offering any kind of Tharpanam to Pitru devatas would make them free from their all sins and gives them a path to reach Deva loka or Vaikunta loka and get moksha.
Pitru Paksha in Ashwin Month (Ashwayuja Masam – Shraddh Paksha), Aadi Amavasai in Tamil Aadi month and Karkidaka Vavubali in Malayalam Karkidaka Masam are the highly meritorious occasions to offer Pitru Tharpanam.
The official website of Sri Veda Vidya Ashram, west Mambalam, Chennai, Tamil Nadu, has got the Tharpanam procedure, types of Tharpanam, what to do and what not to on the day of Tharpanam, Shraddh karma, and Amavasya Tharpanam procedures. You can download Tharpanam procedure in PDF format (English) from this link provided by Sri Veda Vidya Ashram.