Karva Chauth is a day-long fast kept on the fourth day (Chaturthi) of the dark half of the month, in Ashwina by the lunar months that end at the new Moon, and in Kartika by the months that end at the full Moon. Traditionally observed by married women for the well-being of their husbands, and now by many others too, it is kept from sunrise without food or water until the Moon is seen. In the evening there is a puja with a decorated pot (karva), and the fast is broken after the moon is sighted and offered water.
The day is fixed by the Moon, and this is the festival where city matters most. Karva Chauth falls on the civil day on which Chaturthi is in force at moonrise in that city. The moonrise time in the table is computed for each city with the Moon’s centre on the horizon, the convention used by the widely used panchang we compared with. The puja muhurat is the window in the evening before moonrise.
Why do cities differ? Moonrise is at a different clock time in every city, often by an hour or more, and in some years the moon rises on a different civil day. A tithi that covers the moonrise in one city may already have ended at moonrise in another. That is why this page matters more than a single national date, and why the exact moonrise in your own city is the useful figure.
A calm note: fasting is a personal choice. If you have a health condition, are pregnant, nursing or take medicine, speak to your doctor about whether a full fast is right for you, and do not feel that a lighter fast lessens the day. If your family or community keeps a different day or way of observing it, follow them.