The first three days of Navratri are dedicated to Goddess Durga, the next three to Goddess Lakshmi and the last three to Goddess Saraswati.
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According to Brahma Purana, it was on the day of Chaitra Pratipada when Lord Brahma started creating the universe on the behest of Goddess Durga .
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Hence, this day is considered the first day of the year, according to the Hindu calendar.
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It is also believed, that the Matsya avatar of Lord Vishnu was incarnated on the third day of Chaitra Navratri in order to establish Mother Earth.
The ninth day of Chaitra Navratri, which is popularly known as 'Navami or 'Ram Navami' marks the birth of Lord Rama who is one of the great avatars or incarnations of Lord Vishnu.
Goddess Parvati took the form of goddess Durga to visit her parents and kill Mahishasura.