Amavasya is the new-moon tithi that closes each lunar month, when the Moon catches up to the Sun and the night sky turns dark. It is the traditional day for Shraddha, Tarpan and other rites for the ancestors (Pitru), and for vrats such as Somvati Amavasya. Because a tithi is fixed by the angle between the Moon and the Sun rather than by the calendar, the date shifts each month and can differ by location - this calendar lists the upcoming Amavasya dates for your city, each fixed by the tithi running at your local sunrise.
Amavasya is the fifteenth and final tithi of the waning Krishna Paksha - the moonless lunar day when the Moon and the Sun occupy the same 12 degrees of the zodiac. With almost no reflected sunlight reaching us, the Moon is not visible, which is why Amavasya is called the 'no-moon' day.
In the amanta calendar followed across much of India, Amavasya marks the end of the lunar month; in the purnimanta calendar of the north it falls in the middle of the month. Either way it returns about once every 29 to 30 days, and the civil date on which it is observed depends on which tithi is running at sunrise in your location.
Ruled by the Pitru, Amavasya is the classic day to perform Shraddha and Tarpan - offerings of water and food that honour departed forebears. Many families also keep an Amavasya vrat or fast, while new ventures are traditionally avoided on the darkened day. Certain Amavasyas through the year carry special weight:
The named Amavasyas are the same tithi as any other, distinguished only by the lunar month they fall in or the weekday they land on. Their dates are therefore found by exactly the same sunrise rule: a Somvati Amavasya is simply an Amavasya whose chosen civil day happens to be a Monday.
Amavasya observance is kept through the day, so the anchor is sunrise. Kane, History of Dharmasastra Vol. V Part I, p. 73 gives the rule: religious acts prescribed for the day are begun in the morning even when the tithi is mixed with another on that date, so a day-long observance follows the tithi prevailing at sunrise. This calendar therefore lists the civil day whose true sunrise, computed for your latitude, longitude and time zone, falls inside the Amavasya tithi.
Two consequences are worth knowing. First, the same Amavasya can land on different civil dates in two cities, because their sunrises fall on opposite sides of the tithi boundary - which is why a location is asked for rather than assumed. Second, when the tithi is long enough to cover two consecutive sunrises, both days qualify; the calendar lists the earlier one and attaches a note naming both, so the case is visible rather than hidden. The Purnima calendar uses the identical rule, and the two therefore behave the same way.
Amavasya is chiefly observed for ancestral rites - Shraddha and Tarpan for the Pitru - and for vrats such as Somvati Amavasya. It is treated as a day for remembrance, charity and meditation rather than for starting new ventures, which are usually kept for a brighter tithi.
A tithi is defined by the angle between the Moon and the Sun, not by the calendar, so Amavasya returns roughly every 29 to 30 days on a shifting date. The day it is observed also depends on which tithi is running at sunrise where you are, so it can fall a day apart in different cities.
Somvati Amavasya is an Amavasya that falls on a Monday (Somvar). It is considered especially auspicious and is kept as a vrat, particularly by married women praying for the long life and welfare of their family.
By the sunrise rule. The observance is kept on the civil day whose sunrise falls inside the Amavasya tithi, following Kane, History of Dharmasastra V.i p. 73, which governs any rite performed through the day. If the tithi is long enough to cover two consecutive sunrises, the earlier day is listed and a note names both, so you can see that the case arose.
The tithi itself is the same moment everywhere; only its position in the month differs. In the amanta calendar used across much of India, Amavasya ends the lunar month, so the new month begins the next day. In the purnimanta calendar of the north, the month ends at Purnima, so the same Amavasya falls in the middle of a month that carries a different name.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.