Legend of Durga Navratri festival is mentioned in many scriptures including Ramayana, Devi Bhagavatham and several Puranas. Durga Saran Navratri, The festival of nine nights, commemorates the grand triumph of Goddess Durga over the giant demon, Mahishasura.
With his severe penance for Lord Shiva, demon Mahishasura got several boons from Bhola Shankar and started criticizing and destroying innocent people.
To get rid of Mahishasur, the Gods prayed to Goddess Shakti. Goddess Shakti incarnated as Goddess Durga to kill Mahishasur. Durga Devi, astride a lion, fought with the demon in a horrible manner and finally cut off his head and demolished him on the ninth day. As a commemoration of Durgas victory over Mahishasur which also represents the victory of good over evil, Vijaya Dashami is celebrated on the tenth day.