Shardiya Navaratri is a major festival of Hinduism. This festival of worship of Goddess Shakti starts October 17. According to Hindu Panchang, Navratri festival starts from Pratipada of Shukla Paksha of Ashwin month which continues till Navami date. In Navaratri, the nine forms of Maa Durga are worshiped and the devotees observe Mata Rani for nine days. On the first day of Navratri, where there is a law to do Ghatasthapana in the auspicious time, then on the last day, a fast is opened by worshiping Kanyas, the young girls.

Today is the Eighth Navratri where Eighth form of Maa Durga-Maa Mahagauri is worshipped. 

This episode by Himanshu Sharma will tell you the story of Maa Mahagauri along with the special Mantra to worship her.