The tithi is the oldest of the five limbs of the panchanga and the least intuitive, because it is defined by an angle in the sky and not by a sunrise. Amavasya occupies the arc from 348 to 360 degrees of that separation. Since lunar motion quickens and slackens across the month, the arc is crossed in as little as about nineteen hours or as much as about twenty six. The panchanga settles the mismatch by naming each civil day for the tithi in force at its sunrise. A tithi shorter than the interval between two sunrises can therefore be skipped entirely, which the tradition calls a kshaya tithi, and one longer than that interval can carry its name over two civil days, an adhika or vriddhi tithi. Both occur in the ordinary course of a year, and both are why an observance can appear on two dates in the same almanac.
Each tithi is halved again, and each half carries a karana, so the first half of Amavasya carries Chatushpada, the four-footed karana, and the second Naga, the serpent karana that ends the lunar month. Seven of the eleven karanas repeat, cycling eight times through the month, while four are fixed to particular half tithis at the month's opening and close.
At the close of this arc the Moon returns to conjunction with the Sun and the cycle begins again. Through Amavasya the disc shows under one per cent of the disc lit, which is to say effectively none, and the Moon is lost in the Sun's glare. In the amanta reckoning used across much of southern and western India the lunar month ends here, which is the root of the standing discrepancy between regional calendars over which month a given festival belongs to.