On the auspicious day of Maha Navami, which falls on the ninth day of Sharadiya Navratri, Goddess Durga is venerated as Mahisasuramardini, the vanquisher of the buffalo demon, Mahisasura. Mahisasura, the son of asura king Rambha and demon princess Shyamala, had previously waged numerous unsuccessful wars against the gods. In desperation, he undertook severe penance and finally pleased Lord Brahma, who granted him a unique boon.
Mahisasura asked for the gift of immortality, but Lord Brahma denied this request. Believing that he could not be slain by any man, god, demon, or beast, Mahisasura sought protection against such adversaries. His wish was granted, and he subsequently launched a relentless assault on the realms of earth, heaven, and hell, even targeting the heavenly kingdom of Lord Indra.
The distressed gods turned to Lord Brahma, who, in turn, sought help from Lord Vishnu and Lord Shiva. These three divine beings combined their powers to give rise to a new entity – Goddess Durga, an incarnation of Goddess Parvati. She possessed ten arms, each adorned with weapons from various deities, and her celestial mount was a lion, gifted by the lord of the mountains.
A fierce battle ensued between Mahisasura and Goddess Durga, lasting fifteen days. The climax of this epic struggle occurred at the juncture of the ninth and tenth days, known as the 'sandhi' of Navami and Dashami tithis. It was during this moment that Goddess Durga transformed into her warrior form and ultimately triumphed by slaying the buffalo demon on Dashami tithi, symbolizing the victory of good over evil.
Hence, Maha Navami is celebrated as the day when Goddess Durga is worshipped in her form as Mahisasuramardini, commemorating the defeat of the formidable demon Mahisasura and the restoration of divine order.