01What Diwali is
Diwali, or Deepavali, the row of lamps, is the Amavasya of Kartika (Ashwin in the south's reckoning), the darkest night of the month, and the centre of the five days from Dhanteras to Bhai Dooj. It is the best-known festival of India. Lakshmi Puja is made in the evening; houses, streets and temples are lit with lamps; in Bengal and Odisha the same night is Kali Puja; and for many traders it opens the new account year.
02The story told of it
The Ramayana's account is that Rama returned to Ayodhya on this night after fourteen years away and his victory at Lanka, and the people lit rows of lamps to welcome him home. A second thread makes it Lakshmi's night: she is held to have come out of the churning of the ocean and to visit the homes that are clean and lit. Sikhs keep the day as Bandi Chhor Divas, for Guru Hargobind's release from Gwalior fort.
03How it is kept
The house is cleaned through the week before, rangoli is drawn at the door and lamps of clay are set along the walls and windows. In the evening, in Pradosh, Lakshmi is worshipped with Ganesha: the images are set on a red cloth, a kalash is filled, and the puja is made with lamps, flowers, sweets, coins and the family's account books, which traders begin afresh as Chopda or Sharda Puja. Sweets are exchanged with neighbours and family, and lamps are kept burning through the night.
04How the day is chosen
Diwali is kept on the day Amavasya is current in Pradosh, the evening hours after sunset, because Lakshmi Puja belongs to them; when Amavasya covers part of the Pradosh of two days, the second is chosen if it covers at least a ghati there. The muhurat is Pradosh while Vrishabha, a fixed lagna, is rising. Mahanishita, around midnight, and Simha lagna are the hours of the night's worship.
05Diwali in other years