How the date is fixed
Kali Chaudas 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 middle of the night (halfway from sunset to the next sunrise). In the North Indian (purnimanta) calendar the same fortnight is counted in Kartika.
About
Kali Chaudas is the night of the fourteenth tithi of the dark half of Ashwina in amanta reckoning (Kartika Krishna Chaturdashi in the north), the Chaturdashi before Diwali, in October or November. It is a night observance: the day whose middle of the night carries Chaturdashi is chosen, which is often the day before Diwali, while the pre-dawn oil bath of Naraka Chaturdashi can fall on the morning of Diwali itself. In Gujarat and parts of Rajasthan Kali and Hanuman are worshipped that night, and the day is counted among the five days of Diwali.
How it is observed
Worship of Kali and Hanuman at night, and lamps lit at the threshold.