“Amavasya Twamavasya darshasuryendusangama”, says the Amarakosha. Amavasi is the ‘no moon’ day. ‘Ama’ also means ‘with’ and when surya and chandra fall in the same rashi, it is an Amavasi. Once, when Bhrigu cursed Agni, the latter spoke about the importance of Amavasi. The offerings made to Agni in yajnas used to be the food for devas and pitrjanas. The offerings on Puranmasi is taken by the devas and the ones on Amavasi is taken by the pitrjanas, as per Mahabharata Adiparva Chapter 7.