Navratri is most significant festival of India and it has devoted to Maa Durga. There are several myths, stories and legends about the navratri celebrations. As the one legend Mahishasura the powerful demon worshipped Lord Shiva and achieved the power of infinity. So he began to kill and annoy innocent people and set out to win seven lokas.
Intimated by his power, all gods from swargaloka called to Lord Shiva to discipline the demon. Then three Dev Brahma, Vishnu and Shiva united their utmost powers and created a divine being called Shakti the fighter goddess for kill Mahishasura. After the day of killing Mahishasura, the day has celebrated as navratri or Vijaya Dashami.
Other hand, according to one legend lord Rama who wished for release Sita from the command of mighty demon king Ravana prayed goddess Durga in nine aspects for nine days in order to get together the strength and power to kill Ravana. Those nine nights has known as navratri and on the tenth day Rama killed Ravana that day is called Vijayadashmi. There are also many myth or history about Navratri that you can find easily.
Navratri is the sacred and blissful festival of the Hindu civilizations of India. It has observed over a period of nine nights, the festival is famous throughout the country for its religious and social importance. It has devoted to the worship of the divine mother "Durga" in different forms under different names by different people. The navratri significance is much more than just worshipping the nine forms of Maa Durga, Maa Parvati, Maa Lakshmi and Maa Saraswati and the representation of victory over evil. According to popular faith, navratri celebrates the victory of goddess Durga over the evil demon Mahishasura. It is also said that the festival really honors the killing of the great demon king of Lanka, Ravana, by Lord Rama.