How is Mahalaya Paksha Tharpanam different from monthly Amavasya Tharpanam?

Amavasya Tharpanam is performed monthly and carries the merit of a regular ancestral offering. Mahalaya Paksha Tharpanam, performed during the 16-day period of Pitrupaksha, is considered dramatically more meritorious — the Puranas describe it as the one period in the year when the Pitru are most accessible and the offerings reach them most directly. Performing Tharpanam at a sacred tirtha during Mahalaya Paksha is considered the equivalent of years of regular monthly Tharpanam.