Amavasya is the new-moon tithi, the last day of Krishna Paksha, when the Moon is in conjunction with the Sun and not visible. In the Amanta calendar used in South and West India, a new lunar month begins the day after Amavasya.
Amavasya is traditionally dedicated to the ancestors (pitrs): tarpana, shraddha and charity are performed, and it is generally avoided for new beginnings. An Amavasya on Monday is called Somvati Amavasya and one on Saturday Shani Amavasya; Mahalaya Amavasya in Bhadrapada and the Diwali Amavasya in Ashwin are the most important of the year.