Maha Navami, which falls on the ninth day of Sharadiya Navratri, is an important day. Important in the sense that on this day Goddess Durga is worshipped as Mahisasuramardini, the annihilator of the buffalo demon.
Mahisasura, son of Rambha, the king of asuras and demon princess Shyamala, waged wars against devas on several times but got defeated each time. He then started invoking Lord Brahma to grant him a boon. Pleased with his austerity, Lord Brahma appeared before him and told him to ask for a boon.
Mahisasura asked for the boon of immortality which Lord Brahma turned down. As he believed that no woman could kill him, he asked for the boon of no man-god, demon or beast- could kill him. His wish was granted.
He then attacked the three worlds of earth, heaven and hell. He didn’t even spare the Indralok, the kingdom of Lord Indra. Terrified gods approached Lord Brahma, who then went to Lord Vishnu and they all sought help from Lord Shiva.
Then, Lord Brahma, Lord Vishnu and Lord Shiva combined their powers and a new entity took shape. She was Goddess Durga, an incarnation of Goddess Parvati. The ten-armed goddess received weapons from all gods and her vehicle-lion from the lord of the mountains.
Thereafter, a fifteen-day-long battle between Mahisasuar and Goddess Durga ensued.
At the juncture of the ninth and tenth days (‘sandhi’ of Navami and Dashami tithis), Goddess Durga transitioned into her warrior form and killed the buffalo demon on Dashami tithi, symbolizing the triumph of good over evil.
This is why, Goddess Durga is worshipped in the form of Mahisasuramardini on Maha Navami.