Karwa Chauth fasting is done during Krishna Paksha Chaturthi in the Hindu month of Kartik and according to Amanta calendar followed in Gujarat, Maharashtra and Southern India it is Ashwin month which is current during Karwa Chauth. However, it is just the name of the month which differs and in all states Karwa Chauth is observed on the same day.
Karwa Chauth coincides with Sankashti Chaturthi a fasting day observed for Lord Ganesha. The fasting of Karwa Chauth and its rituals are observed by married women for the long life of their husband. Married women worship Lord Shiva and His family including Lord Ganesha and break the fast only after sighting and making the offerings to the moon. The fasting of Karwa Chauth is strict and observed without taking any food or even a drop of water after sunrise till the sighting of the moon in the night.
Karwa Chauth day is also known as Karak Chaturthi (करक चतुर्थी). Karwa or Karak refers to the earthen pot through which water offering, known as Argha (अर्घ), is made to the moon. Karwa is very significant during Puja and it is also given as Dan to the Brahmin or any eligible woman.
Compare to South Indian states, Karwa Chauth is more popular in North Indian states. After four days of Karwa Chauth, Ahoi Ashtami Vrat is observed for the wellbeing of sons.