Purnima is the fifteenth tithi, the full moon, when the Moon stands opposite the Sun and the disc is completely lit. It closes the bright fortnight and is a Purna tithi, the complete day, read in the panchanga as broadly auspicious. Its presiding deity is the Moon itself, Chandra or Soma. Many of the most important festivals of the year fall on Purnima: Guru Purnima honours the teacher, Buddha Purnima marks the birth of the Buddha, Sharad Purnima the autumn full moon, and the full moons of Kartik and Magha are kept for bathing and charity. Fasting and pilgrimage are common on the day. These are the conventions of the calendar and its festivals; the astronomical basis is the exact opposition of Moon and Sun that produces the fully illuminated disc.
Because Purnima is defined by exact opposition of Sun and Moon, it is also the necessary condition for a lunar eclipse, which can occur only when the Moon is full and additionally crosses close to one of the two nodal points where its orbital path intersects the ecliptic; most Purnimas pass without any eclipse since the Moon usually passes above or below the Earth's shadow, but when the alignment is close enough, the astronomical basis of the tithi and the astronomical basis of the eclipse are, quite literally, the same alignment observed at two different levels of precision. The four-month period of Chaturmasya, during which certain classes of activity are traditionally deferred, is bounded at its start and close by specific Purnimas, tying this fifteenth tithi to the wider structure of the ritual year beyond its individual monthly festivals.