Ashtaka Tarpanam is a mandatory ritual. It occurs on the Krishna Paksha Ashtami in the month of Magha (this usually falls after the Thai Amavasya). The month of Magha is named after the Magha Nakshatra, and the presiding deities of that Nakshatra are the Pitrus (ancestors). Therefore, the month of Magha has a special connection to Pitrus. Furthermore, the Krishna Paksha is considered the Pitru Paksha. Since Ashtami is a significant Tithi for ancestors, the Krishna Paksha Ashtami in Magha month is very special for them. This is called “Ashtaka.” You should consult the Panchangam and elders to perform this essential ritual every year.