Navratri is one of the most significant festivals to Hindus in the year. The word Navratri clearly is known as 9 nights.
The legend associated with Navratri speaks about the great battle that took place between the powerful demon Mahishasura and Goddess Durga. Mahishasura was blessed with immortality by Lord Brahma under one condition that the powerful Mahishasura could be defeated only by a woman. With this confidence the demon attacked earth, heaven and hell. Since only a woman could defeat him, even the Gods didn’t stand a chance against him. The worried Gods prayed to Lord Brahma, Lord Vishnu and Lord Shiva to help them defeat their worst enemy. Looking at the helpless Gods, Lord Vishu took the decision to create a woman to defeat Mahishasura since as per Lord Brahma’s boon, none but only a woman can defeat the demon. Lord Shiva and Lord Brahma put all their powers together in the woman Lord Vishnu, had created to demolish Mahishasura. It is believed that Goddess Ambe is a reincarnation of Goddess Parvati.

Maa fought with Mahishasura for 10 long days. During the fight, the clever Mahishasura kept changing his form to confuse his opponent Goddess Durga. Ultimately, when the demon took the form of a buffalo, the Goddess Durga pierced his chest with her trishul killing him instantly.
So, on each day of Navaratri different avatars of Goddess Durga is worshipped. On the first day, people worship Goddess Shailputri while Goddess Brahmacharini is worshipped on the second day. On the third day people pay homage to Goddess Chandraghanta; on the fourth day Goddess Kushmanda is worshipped; on the fifth day Goddess Skandamata is worshipped; on the sixth day Goddess Katyayani is worshipped; on the seventh day Goddess Kaalratri is worshipped; on the eighth day Goddess Mahagauri is worshipped and on the last and final day people worship the Goddess Siddhidatri.