How the date is fixed
Karwa Chauth falls on the fourth day of the dark half of the lunar month Ashwina (Krishna Chaturthi). The date is the day on which that tithi is running at moonrise. In the North Indian (purnimanta) calendar the same fortnight is counted in Kartika.
About
Karwa Chauth falls on the fourth tithi of the dark half of Ashwina in amanta reckoning, called Kartika Krishna Chaturthi in the north, in October or November. It is a Sankashti Chaturthi, so the governing moment is moonrise: the day on which Chaturthi is present at moonrise is chosen, and the fast ends when the Moon is seen. The Moon rises late on this tithi, usually between 8 and 9 pm, and panchangs print the local moonrise for the city. The vow is kept chiefly in Punjab, Haryana, Delhi and western Uttar Pradesh.
How it is observed
Sargi before dawn, a day without food or water, the karwa story in the evening, and the fast broken after seeing the Moon through a sieve.