Amavasya is the thirtieth and final tithi, the new moon, when the Moon stands in conjunction with the Sun and its disc is dark and unseen. It closes the dark fortnight and the lunar month. Its presiding deities are traditionally the Pitris, the ancestral fathers, and the day is the chief occasion for shraddha and tarpana, the rites of offering to the ancestors, the fortnight of Pitru Paksha culminates on an Amavasya. The new moon of Kartik is Diwali, kept with lamps against the darkest night. In electional astrology the panchanga generally avoids Amavasya for auspicious beginnings, treating it as a day for ancestral and inward observance rather than outward ventures. These are the conventions of the calendar tradition; the astronomical basis is the exact conjunction of Moon and Sun that leaves the night moonless.
Pitru Paksha, the fortnight of ancestral rites that culminates on this Amavasya, is fixed to the dark fortnight of the lunar month of Bhadrapada or, in some regional reckonings, Ashvin, and its sixteen days each correspond to a different possible tithi of death, so a family observes shraddha on whichever day within the fortnight matches the deceased relative's actual date of death, with the Amavasya itself serving as a catch-all day for ancestors whose precise date of death is unknown or unobservable. Diwali's date, the Amavasya of Kartik, is calculated according to which tithi prevails at dusk, since the lighting of lamps against the moonless dark is the observance's defining act, making this one of several major festivals whose correct calendar date depends on identifying the tithi at a specific time of day rather than at sunrise.