01What Purnima is
Purnima is the full moon, the fifteenth and last tithi of the bright half, Shukla paksha, of each lunar month. It comes about every twenty-nine and a half days, so a year has twelve, or thirteen when it has an adhik month. Calendars that end the month at the full moon (purnimanta, used across North India) close each month on it, and several of the best known festivals of the year fall on one: Guru Purnima, Buddha Purnima, Sharad Purnima, Kartik Purnima and Holi among them. The tradition treats every Purnima as a day for Vishnu, and in many homes for the Moon as well.
02How the day and time are worked out
A tithi does not begin at midnight, so a Purnima can start at any hour and often spans parts of two civil days. For an ordinary month this page lists Purnima Vrat on the day whose sunset falls inside the Purnima tithi, because the vrat belongs to the night of the full moon, when the Moon is worshipped after it rises. Guru, Buddha and Kartik Purnima follow the tithi at sunrise, as the panchangs keep them; Sharad Purnima follows sunset, since its observance is at night. Each entry carries the moonrise for the city you choose, so a family can plan the evening puja.
03How it is kept
Many households keep Purnima with the Satyanarayana katha: a puja to Vishnu in his form as Satyanarayana, the reading of the account of the vrat in its five chapters, and prasad of sheera or panjiri shared with family and neighbours. Bathing in a river, and above all at a confluence such as Prayag, is an old Purnima custom, followed by charity. Some keep a fast through the day and eat after offering water to the risen moon. Others simply light a lamp in the evening. The tradition leaves the strictness to the household, and regional customs differ from one month to the next.
04The rest of the calendar
