Navratri means nine nights however, we use the word Navratri here to describe the worship of Goddess Durga in her nine avatars (forms). The festival of Navratri is held during the start of full moon in the month of Aso as per Gujarati Hindu calendar. This differs with the English calendar and the celebrations vary between the months of September and November depending on the alignments of the lunar calendar.
Supreme Mother Goddess Durga is worshipped in the form of Kali, Lakshmi and Saraswati. During Navratri the first three days of celebrations are worship to Goddess Kali who is the destroyer of all our impurities, the second three days of worship are to Goddess Lakshmi the giver of inexhaustible wealth. The last three days of Navratri are worship of Goddess Saraswati the giver of knowledge. The nine avatars of Mother Durga are: Maa Shaiputri, Mata Bhrahmacharini, Mata Chandraghanta, Mata Kushmanda, Maa Shanda Mata, Maa Katyayani, Mata Kalratri, Mata Maha Gauri, and Mata Siddhidatri.