Viswamitra

"Viswamitra said,

'Listen to a high mystery that is unknown to the generality of men and that is connected with religion. He who

1. offers the Pitris rice boiled in sugared milk, 
2. sitting with face directed to the south at noontide in the shade caused by an elephant's body, 
3. in the month of Bhadrapada, 
4. under the constellation Magha, acquires great merits. 

Listen to what those merits are. The man who makes such an offering to the Pitris under such circumstances, is regarded as performing a great Sraddha each year for thirteen years in succession.'

http://www.sacred-texts.com/hin/m13/m13b091.htm