The main goddess that is worshipped on Karva Chauth vrat is Goddess Parvati. But there is also a temple dedicated to Chauth Mata in Rajasthan. This manifestation of Mother Goddess is also worshipped during the Karwa Chauth puja.
Majority of the people offer prayers to Goddess Parvati on the day as it is believed that She observed the vrat on advice of Shiva for a happy and peaceful married life.
Chauth Mata is also a manifestation of Mother Goddess Shakti. Worshipping this form is equal to worshipping Goddess Parvati.
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