Diwali

Diwali's date is set by the Hindu lunisolar calendar, and the calendar exists in two variants that count the same month differently, which is why Indian regional dates for festivals sometimes disagree.

The festival falls on Amavasya, the new moon, of the month Kartika. The complication is that the two reckonings define a month differently. In the purnimanta system, used across north India, a month runs from full moon to full moon. In the amanta system, used in the south and west, it runs from new moon to new moon. The same lunar day therefore carries different month names in different regions, and festivals fixed by month-and-day can differ.

For Diwali the systems happen to converge on the same night, because the new moon is a boundary in one and a midpoint in the other, and both land on the same darkness. Other festivals are not so lucky, and Krishna's birthday and Maha Shivaratri fall in differently-named months depending on the region.

The Hindu calendar intercalates like the Hebrew one, adding an adhika masa, an extra month, roughly every 32 to 33 months, which keeps Kartika in the autumn. Diwali consequently stays between mid October and mid November.

There is a further layer. The traditional calculations use the sidereal zodiac, measured against the fixed stars, rather than the tropical one measured from the equinox, and the two diverge by about 24 degrees because of precession. Different almanacs use slightly different values for that offset, which produces occasional one-day disagreements between panchangas.

Calling Diwali the Hindu festival of lights is true and flattens a great deal.

It runs across five days with distinct observances, Dhanteras, then Naraka Chaturdashi, then the main night, then a day variously kept as Govardhan Puja or the Gujarati new year, then Bhai Dooj for brothers and sisters, and which of these are emphasised varies sharply by region.

The story attached also varies. North India predominantly connects it to the return of Rama to Ayodhya after fourteen years of exile, with lamps lit to welcome him. In Bengal the night is given to Kali. In Gujarat and much of the mercantile west it is primarily the worship of Lakshmi and the opening of new account books, and the business year begins with it. In the south the emphasis falls on Krishna's defeat of Narakasura. These are not versions of one narrative; they are different festivals sharing a date and a name.

It is also not only Hindu. Jains keep the same night as the anniversary of Mahavira's attainment of liberation. Sikhs keep Bandi Chhor Divas, commemorating the release of Guru Hargobind from imprisonment at Gwalior along with fifty-two princes, and the Golden Temple is illuminated. Some Newar Buddhists in Nepal observe it. These communities are not celebrating a Hindu festival by courtesy; each has its own reason for the date.

The firework tradition is recent enough to be documented and is now a serious urban air quality problem, addressed by Indian court orders restricting the type and hours of use.