Navratri is one of the most celebrated of all Hindu festivals. “Nav” means nine, and “ratri” means night; thus, Navratri festival means a festival spread over nine nights.
Though Navratri is a mix of various themes, the common factor is the destruction of evil or the victory of good over evil.
In the Chaitra Navratri, Demon Mahishasura (who had defeated all the Gods) was ultimately killed by Goddess Durga. After the Gods were defeated, they approached Brahma, Vishnu, and Mahesh, whose collective energy gave rise to shakti or power in the form of Goddess Durga. They appealed to her to kill the demon.
In the Chaitra Navratri, the 9th day is celebrated as Ram Navami, the day Lord Rama was born.
In the Sharad Navratri, the 10th day is celebrated as Vijayadashami or Dusshera, the day when the demon king Ravana was killed by Lord Rama.