1. Goddess Durga Triumphs Over Demon MahishasuraMystically, it is believed that Demon Mahishasura received a boon where no male can kill him. Proud and arrogant, he started to create a havoc and Goddess Durga battled with him for 9 days before defeating him on the 10th day, Vijayadashami. Goddess Durga's victory is seen as the defeat of ego and evil by dharma, hence the days of the battle and victory celebrated as Navarathri and Vijayadashami.
2. 9 Different Avatars of Shakti (Supreme Source of Power)According to Hinduism, there are 3 different forms of Shakti: Mahasaraswati, Mahalakshmi and Mahakali. They are the 'shaktis' of Lord Brahma, Lord Vishnu, and Lord Shiva respectively, without whom the Gods will lose their power.
Each of these 3 forms of Shakti takes 3 more manifestations and are worshipped on the 9 days of Navarathri. The first 3 days are dedicated to Goddess Durga, the next 3 days to Goddess Lakshmi and the last 3 days to Goddess Saraswati.
3. Lord Rama Seeking Goddess Durga's BlessingsAnother reason why some believe Navarathri is celebrated for 9 days is that Lord Rama sought Goddess Durga's blessings to kill the ten-headed Ravana. Hence, Rama prayed and fasted for 9 days before winning the battle on the 10th day which is widely known as the Dusshera in many parts of India.
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