Saturday.
Prime Minister Narendra Modi greeted the nation on the occassion-"My greetings to everyone as the auspicious Navratri commences," Modi tweeted.
The nine day fast festival of Navratri is celebrated in the honour of Goddess Durga. The word Navaratri means 'nine nights' in Sanskrit. During these nine nights and ten days, nine forms of Devi are worshipped.
The tenth day is commonly referred to as Vijayadashami or "Dussehra". It marks the victory of Durga over the demon Mahishasura. The goddess fought with evil for ten days and nine nights. (With inputs from Wikipedia)