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. Purnima occupies the arc from 168 to 180 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 Purnima carries Vishti, the karana also called Bhadra, and the second Bava, first of the seven movable karanas. Vishti is the one karana the muhurta literature marks as unfit for beginnings, and its presence in the first half is why this tithi is read with more care than its neighbours. 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 stands opposite the Sun and the month turns. Through Purnima itself the disc shows over ninety-nine per cent of the disc lit, a disc the eye reads as full. In the purnimanta reckoning used across much of northern India the lunar month ends here rather than at the new Moon, which is why the same tithi can sit in differently named months in two almanacs printed the same year.