Sharad Purnima, the full Moon of autumn, is regarded as the brightest and most beautiful of the year. It comes when the monsoon has ended and the sky is clear, and in the tradition the Moon is believed to be complete in all her sixteen phases on this night. The day is also called Kojagara Purnima, Kaumudi Purnima and, in Bengal, Lakshmi Puja.
Month and tithi
It is the full Moon (Purnima) of Ashwina, the bright fortnight that follows the Navratri nights. The tithi is the fifteenth, explained on the Purnima page. The festival is also observed by many as Valmiki Jayanti, the birth anniversary of the sage Valmiki, on the same day.
The traditions of the night
The best known custom is to cook kheer, rice cooked in milk with sugar and dry fruit, and leave it outside in the moonlight through the night, then eat it as prasad. Folk belief holds that the rays of this full Moon carry a special freshness. This is a tradition of the night, not a medical claim, and it is a pleasant way to mark the season.
In Gujarat the night is kept with Raas and Garba, and in Braj it is remembered as the night of Krishna’s Raas Leela with the gopis, when he is said to have played the flute under a full Moon. In Bengal, Odisha and Mithila, the festival is Kojagara: the name is explained as the question “Ko jagarti?”, which means “Who is awake?”, and the goddess Lakshmi is said to go through the night asking this question and to bless those who are awake. Families keep a vigil, play games and sing, and worship Lakshmi at midnight. The Diwali Lakshmi puja page describes the ritual.
How the day is fixed
Because the Moon is the heart of the festival, the rule asks about midnight. The rule used here follows Drik Panchang: the festival is on the civil day on which Purnima is in force at Nishita, the short window around the middle of the night; if it is so on two days, the second is taken. The table gives that date, the moonrise on that day and the times at which Purnima begins and ends. On a night when Purnima is still in force at moonrise, the kheer can go out at once.
Why cities differ
Purnima begins and ends at one instant worldwide, but midnight and moonrise are at different clock times in each city. A Purnima that ends between the midnight of two neighbouring cities gives them different dates, and a city far to the east or west may be a day ahead or behind India. The any-city tool gives the result for other places, and the Purnima vrat page lists the monthly Purnimas.
Practical notes
Keep the kheer covered with a thin cloth so that insects and dust stay out, and bring it indoors before the morning. If the sky is cloudy, the offering is still made. If the Moon is not visible where you live because of cloud or city light, the night is not lost: the tradition is about the night, the family and the food.
A calm note
Some families keep a fast through the day and break it with the kheer; it is a choice, not an obligation. Anyone with diabetes, on medicine or with a health condition should eat sweets as their doctor advises. Follow your own family and temple for the day.