Karva is another word for ‘pot’ (a small earthen pot of water) and chauth means ‘fourth’ in Hindi (a reference to the fact that the festival falls on the fourth day of the dark-fortnight, or krishna paksh, of the month of Kartik). It is uncertain how the festival originated and how it came to be celebrated only in the northwestern part of the Indian subcontinent. One hypothesis is that military campaigns and long-distance travel usually resumed around the time of the festival, as the area dried and numerous rivers of the region (see Sapta Sindhu) subsided from the effects of the monsoon.
Women observed the fast to pray for the safety of their husbands at this time as they ventured away from home. The festival coincides with the wheat-sowing time (i.e., the beginning of the Rabi crop cycle). Big earthen pots in which wheat is stored are sometimes called Karvas, so the fast may have begun as a prayer for a good harvest in this predominantly wheat-eating region.
There is another story about the origin of this festival. Earlier, girls sometimes barely teenagers used to get married, go and live with their in-laws in very remote villages. Everyone would be a stranger there for the new bride. In case she had any problems with her husband or in-laws, she would have no one to talk to or seek support from. Her own parents and relatives would be quite far and unreachable. Telephones, buses and trains were not heard of in those days. People had to walk almost a whole day to go from one place to other.
Once the girl left her parent’s home for in-laws, she might not be back before long. Thus the custom started that, at the time of marriage, when bride would reach her in-laws, she would befriend another woman there who would be her friend (kangan-saheli) or sister (dharam-behn) for life. It would be like god-friends or god-sisters. Their friendship would be sanctified through a small Hindu ceremony right during the marriage. The bride’s friend would usually be of the same age (or slightly older), married into the same village (so that she would not go away) and not directly related to her in-laws (so there was no conflict of interest later). Emotionally and psychologically, it would be very healthy and comforting for the bride to have her own ‘relative’ near her.Once the bride and this woman had become god-friends or god-sisters, they would recognize their relation as such. They would treat each other like real sisters. During any issues later in life, involving even the husband or in-laws, these women would be able to confidently talk or seek help from each other.
Moreover, the bride’s parents would treat her friend just like their own daughter. Thus Karva Chauth started as a festival to celebrate this special bond of friendship between the brides and their god-friends . The notion of praying and fasting for the husband came much later and is secondary. It was probably added, along with other mythical tales, to enhance the meaning of the festival. In any case, husbands would always be associated with this festival, because the day of starting this friendship between two god-sisters was essentially the day of bride’s marriage to him.
Hence praying and fasting for him by his wife during a celebration of her relationship with the god-friend would seem quite logical. A few days before Karva Chauth, married women would buy new karvas (spherical clay pots) — 7″-9″ in diameter and 2-3 litres capacity—and paint them on the outside with beautiful designs. Inside they would put bangles and ribbons, home-made candy and sweets, make-up items, and small clothes. The women would then visit each other on the day of Karva Chauth and exchange these karvas.
Reference : http://en.wikipedia.org/wiki/Karva_Chauth