Karva chauth usually celebrated in the karthika masam krishna paksh chaturthi. Mostly it comes first week of november.
Married women perform this vrath for their husband's longevity on this auspecious day.
In purana's its said that a women called karwa who saved her husband's life by fighting against crocodile. She stopped yamraj of taking away her husband's life
Women applies mehendi designs and worships to god for thier husband's good health and longlivity.
Married women worships ther husbands with diya and unmarried ladies worships to god for good, handsome and best husband in their life.
They worships to moon and listen to the karwa chauth story. This is a very important festival in north india and many states announce official holiday for women on this day.
From morning they do fasting and after giving argh to the moon they leave fast by having sweet with their husband.