The fight to save Goddess Sita Devi lasted for 10 days and on the tenth day of Ashwayuja month, Lord Sri Rama managed to kill Ravanasura with the help of 'Divyastra' given by Goddess Durga Devi. The day when Lord Sri Ram killed Ravanasura was the Dashami Tithi (tenth day) of Ashwayuja month and hence it is called Vijaya Dashami.