Karva Chauth Vrat is kept by married women for the long lives of their husbands. Here is Karva Chauth Vrat Katha in Hindi describing how the vrat should be kept, Vrat story as well as Karva Chauth Vrat Ujman. The Hindu women who celebrate Karva Chauth fast through the morning and then break the fast in the evening after observing moon through a sieve. The site has more resources on the Karva Chauth Vrat.

http://www.karwachauth.com/karwa-chauth-vrat-katha.html