Dates differ according to gregorian calendar which is tabular and not based according to constellations. Karva Chauth, like many Hindu festivals, is based on the lunisolar calendar which accounts for all astronomical positions, especially positions of the moon which is used as a marker to calculate important dates.
On Karva Chauth women, especially in Northern India, who are married fast from sunrise to moonrise for the safety and longevity of their husbands. The Karva Chauth fast is traditionally celebrated in the states of Rajasthan, Uttar Pradesh, Himachal Pradesh, Haryana, Punjab, Delhi and as Atla Tadde in Andhra Pradesh. The festival falls on the fourth day after the full moon, in the Hindu lunisolar calendar month of Kartik. Sometimes, unmarried women join the fast for their fiancés.