Karva Chauth is the festival celebrated in north India. This is a one day festival where married women fast the entire day from sunrise to moon rise. The reason for fasting is the health, longevity and well being of their husbands. Sometimes unmarried woman also fast for their fiances or desired husbands.
According to Hindu calendar, this Karva Chauth is celebrated on the fourth day after the full moon in the month of Kartik.
The History of this festival is unknown and also it is not known why this festival is celebrated only in north India.
As per the ritual, the women wakes up early at dawn and eat and drink before sunrise. After the sunrise till the moon-rise women observe complete fast. some women do not even drink water throughout the day. Traditionally on this day women does no house work but meet relatives and friends. At the moon rise, Women break their fast by looking to the moon first through a sieve and then at her husband through the same sieve.
Today this is considered as a romantic festival as it signifies the love of a wife towards her husband.
Source : http://en.wikipedia.org/wiki/Karva_Chauth