Diwali is observed on the Amavasya — the new moon — of Kartika, the darkest night of the lunar year. That choice is the whole meaning of the festival: the lamp matters precisely because the night is moonless. Lakshmi, the goddess of prosperity and wellbeing, is invited into a home that has been cleaned, repaired and lit.

The festival carries different memories in different regions, and all of them are held to be true at once. Northern India remembers Rama's return to Ayodhya after fourteen years of exile, when the city lit rows of lamps to guide him home. Southern India remembers Krishna's defeat of Narakasura at dawn. Jains observe the nirvana of Mahavira, and Sikhs keep Bandi Chhor Divas, the day Guru Hargobind walked free from Gwalior Fort with fifty-two princes.

Lakshmi Puja is performed in the evening, during pradosh kaal — the window that begins at sunset — and a sthir (fixed) lagna is preferred, so that the wealth invited in is wealth that stays.

Several stories meet on this one new-moon night. In the North it is the night Rama returned to Ayodhya after fourteen years of exile and the defeat of Ravana; the city lit rows of lamps to bring him home through the dark. It is also the night Lakshmi arose from the churning of the ocean, and — in another telling — chose Vishnu as her consort, so she is welcomed into clean, lit homes. In the South the day before marks Krishna's slaying of the demon Narakasura. The single thread through all of them is a lamp answering the darkest night of the year.

Diwali falls on the Amavasya of Kartik — the darkest night just after the monsoon, as winter sets in. Lamps and a thoroughly cleaned home answer that darkness practically as well as symbolically: warmth as the cold begins, and light where there is least of it. It closes the agricultural year in much of India — the harvest is in — which is why traders open fresh account books (bahi-khata) and the season of buying and giving begins here.

Diwali is the victory of light over darkness and of knowledge over ignorance — but the light is meant to be lit within, not only in the doorway. Lakshmi, whom the festival welcomes, is not merely money; she is shri, the auspiciousness and grace that settles only where there is cleanliness, order and generosity. The lamp asks nothing of the darkness except that a single flame be kindled against it.

When the devas and asuras churned the ocean of milk, Lakshmi rose from it seated on a lotus, and of all the beings present she chose Vishnu. Prosperity, the story says, goes where dharma already stands — it is not seized but received.

The night is also Rama's homecoming. Fourteen years after he walked into exile, Ayodhya learned he was returning. The citizens had no time to prepare a welcome worthy of him, so they did the one thing they could: they lit every lamp they owned and set them along the roads, the riverbanks and the rooftops, until the moonless night was brighter than a full moon. It is said Rama entered a city that had made its own light.