How the date is fixed
Naraka Chaturdashi falls on the fourteenth day of the dark half of the lunar month Ashwina (Krishna Chaturdashi). The date is the day on which that tithi is running at the rising of the waning Moon before dawn. In the North Indian (purnimanta) calendar the same fortnight is counted in Kartika.
About
Naraka Chaturdashi is the fourteenth tithi of the dark half of Ashwina in amanta reckoning, the day between Dhanteras and Diwali, in October or November. The abhyanga snan (oil bath) is taken in the last part of the night, after the waning Moon has risen and before sunrise, so the day on which Chaturdashi is present at that pre-dawn moonrise is chosen. In many years this puts Naraka Chaturdashi on the morning of Diwali itself, while the Chhoti Diwali lamps are lit the evening before. Kali Chaudas, the Chaturdashi night kept in Gujarat, has its own entry; Bengal keeps Bhoot Chaturdashi with fourteen lamps, and Goa burns effigies of Narakasura.
How it is observed
An oil bath before dawn, lamps lit in the evening, and in Bengal fourteen lamps for fourteen generations of ancestors.