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Kedara Gauri Vrat is mainly observed in Southern Indian states especially in Tamil Nadu. It is also known as Kedara Vratham. It is observed on the day of Deepavali Amavasya and coincides with Lakshmi Puja day during Diwali.
In some families, Kedara Gauri Vrat is observed for 21 days. The twenty-one days long fasting ends on the day of Deepavali Amavasya. However, most people observe a single day fasting on Kedar Gauri Vrat day. It is an important fasting day for the devotees of Lord Shiva.
Bhringi Rishi was a great devotee of Lord Shiva. However, the great sage had trust in Lord Shiva only and used to ignore His Shakti. This annoyed Shakti Devi of Lord Shiva and She removed the energy from the body of sage Bhringi. The removed energy was nothing but the Goddess Gauri herself.
The removed energy wanted to be part of Lord Shiva's body. She observed the Kedara Vrat to please Lord Shiva. The austerities of removed Shakti, pleased Lord Shiva so much that He gave the left part of His body to removed Shakti. The resulted form of Lord Shiva and Goddess Shakti was known as Ardhanarishvara (अर्धनारीश्वर).
As Goddess Gauri Herself observed this fasting to please Lord Shiva, the Vrat is known as Kedara Gauri Vrat.